The accuracy of early cost estimates in engineering and construction projects is extremely important to both feasibility study and evaluation of design alternatives. In particular, to evaluate alternatives at the design stage, quick and accurate decision-making is needed under a limited definition of scope and constraints in available information and time. This study suggests an approximate cost estimation model for PSC Beam bridges based on the quantity of standard work. For model development, this study analyzed cost structure and proportion of cost items through bill of materials and quantities, identified 8 representative work items (Manufacturing PSC Beam, material, rebar fabrication/placing, supporting post/scaffolding, form work, slab waterproofing, concrete placing, deck finishing), and established approaches for quantify estimating on the work items. By utilizing the characteristics of bridges, the suggested model supports quick cost estimation with only three input values (length of span, total length of bridge, and width). The developed model is applied to 39 real projects for validation. The result of the application showed the accuracy with estimation error less than 4.04%.