When we design the plant for new product or expansion, the capacity of plant is one of the important factors which we should decide. This paper develops the models for optimal capacity considering the economies of scale in construction, operation, replacement cost and a possible importation when demand is growing at a constant geometric growth rate. The models by two criteria - equal-cycle, B/C ratio - contain all the possible cash flows generated from plant construction. And the optimality conditions for each model are derived. These models provide an optimal time-path for the initial plant and expansion. The results and comparison of optimal policies by two criteria are shown by computational experience from the real data.