This paper introduces both domestic and overseas methods to select conductor of overhead line, and the principles and methods of how to calculate the conductor current capacity. Then it presents the main factors which determine the line capacity. It also contrasts and analyzes the calculating parameters and limits about line transmission capacity on design handbooks and rules of domestic electric power industry, and recommends the current-carrying capacity of common conductors of Jiangsu 220 kV and 500 kV Grid. In the end, it offers some proposals for how to improve the line capacity.