In developing phase of a hydrocarbon reservoir and planning for drilling the production wells, it is necessary to drill these wells in an appropriate spacing to achieve maximum economic revenues, during reservoir life span. Traditionally, the optimum well spacing has been determined graphically from a plot of economic return versus spacing among the wells. This paper derives an equation to solve this problem directly without the plot and presents an illustrative example for its application in a recent developed Iranian onshore oil field. Then we optimize achieved function using a genetic algorithm (GA) approach with an initial random population with 20 chromosomes and 100 generation number as main optimization engine in order to finding optimum well spacing versus maximum project NPV. Our procedure show a well-matched results with figures that calculated by project contractor achieving from a trial and error process during defining the field developing scenarios. Introduction At the beginning of a hydrocarbon field development plan, when the field's geometry and the amount of proven, probable and possible reserves has been delineated, one of the most important variables to be decided for proper development are optimum number of wells and spacing among them. Well spacing is generally defined as maximum area of the resource reservoir that can be efficiently and economically drained by one well1. Spacing is accomplished by an order of the Colorado Oil and Gas Conservation Commission (COGCC) for a particular geographic area1. The propose of well spacing is prevent waste, avoid the drilling of unnecessary wells, and protects the rights of reserves owners1. Muskat2discussed the well spacing problem from two points of view: the physical ultimate recovery and the economic ultimate recovery. From the physical standpoint there are a minimum number of wells Wm required to achieve maximum extraction. Increasing the number of wells beyond this number would not increase the ultimate primary extraction. From the economic ultimate recovery standpoint, giving no time limit for a reservoir development project's life, it can be stated, pure theoretically that at one extreme a few wells can drain the whole reservoir, and at the other extreme, an unnecessary high number of wells could effectively drain the reservoir more rapidly, but at a high cost. In either case, the project's economic return would be negatively affected. Between these two extremes there ought to be an optimum number of wells Wo that would yield maximum economic return. This concept applies equally for vertical wells and/or horizontal wells3. Some authors developed graphical methods for estimating proper well spacing. The optimum well spacing has been determined graphically from a plot of economic return versus well spacing as proposed by Muskat2 (1949).Garaicochea and Acuna4 (1978) approach for determining optimum well spacing consists of predicting performance of the reservoir under various spacing schemes graphically. Another possibility for calculating the optimum well spacing is to prepare a cross plot of net present value vs. number of wells using as a parameter oil price as illustrated by Martinez5 (1975). A method to determine the optimum well spacing straightforward without a plot was presented by Tokunaga and Hise6 (1966). This method, however, assumes the production rate of all wells to remain constant over life (no decline). Corrie3 (2001) propose an analytical solution that the well's initial production rate to decline over life of reservoir (immediately after starting production).The method presented in this paper will cover three main production stages (build up, plateau and decline) of a real reservoir for finding proper well spacing. Well spacing principles As mentioned before well spacing has two different features. In this article we want to underline the economic aspect of problem. The problem is one of determining the optimum number of wells to drill and the accurate spacing among them, to get maximum economic profit. The following considerations highlight the subject: