This paper describes computational algorithms and a computer program for optimum determinations of wire conductor sizes of radial or tree-type distibution lones with given constraints. Here, The objective function is defined as the total summation of the volume or weight of respective conductor materials required for buildingup the entire distributing system. Four categories of constraints are applied to the obiective function. That is, on the respective load points constraint is imposed by a specified voltage drop limit, and the respective line elements are capable of carrying the current safely(safety current) and also must maintain the minimum thickness in viewpoint of mechanical strength and legal requirements. And finally, the conductor sizes have to be selected among the standardized size levels of the products. These kinds of optimization problems cannot be solved by the ordinary optimization tediniques such as the Linear Programming Method, SUMT Technique, etc. This paper, therefore, successfully devised the powerful alorithms to solve the problem, using the particular properties or characteristics ingerent to the radial or tree-type distribution system. The computer program developed from the algorithms was applied to several sample systems and shown to be exact and very efficient.