The principle of construction of stochastic and deterministic mathematical models for prediction of operating modes of gas pumping units at compressor stations of main gas pipelines and their optimization in conditions of incomplete loading of the gas transmission system is proposed. Calculation of the parameters of the technological mode of operation of the compressor stations of gas pumping units is based on the use of combined characteristics of centrifugal superchargers, the processing of which by means of mathematical statistics allowed to obtain analytical expressions for the characteristics models. The calculation method can be implemented for forecasting the operation modes of the compressor station in conditions of both single-stage and multi-stage compression of gas. The source data for the calculation comprise the upstream and downstream pressure and temperature of gas, the volume of pumping and physical properties of the gas. In order to predict operating modes of compressor stations, the optimal distribution of load between workshops equipped with multi-type gas-pumping units is carried out provided that the energy consumption is reduced to compress the given volume of gas under known boundary conditions. The principle of constructing an optimality criterion for conditions of incomplete loading of the gas transmission system is shown. To illustrate the proposed method of optimizing the operation modes of compressor stations under incomplete loading, we give an example of the calculation of optimal operating modes of a hypothetical two-seater compressor station with various types of aggregates. The calculation is based on the construction of analytical mathematical models of their characteristics on the criterion of minimum capacity for gas compression at a given output and upstream and down stream pressures at the station.