Abstarct: In this thesis, sizing optimization of steel portal frxames with tapered members by Genetics Algorithm on the basis of AISC code has been studied. The goal of this research is to develop a user friendly computer program that can be able to analysis, design and optimization of nonprismatic portal gable frxames. This program is baxsed on current hypothesis in construction of this type of structure e.g. single story, plane frxame with nonprismatic member. Each member of the frxame consists of 5 design parameters, namely, height of web at the head of member, height of web at the end of member, Thickness of web, Thickness of flange and breadth of flange that will be optimized during algorithm process. Finally the optimum module type of frxame, that yielding minimum weight of structure will be provided to user. In exposition of the thesis, after introduction of chronicle from optimization of tapered member, genetic algorithm was explained, and then the terms and methods of analysis and design of nonprismatic frxames were surveyed. Finally, accuracy and worth out put of the program were presented.