Abstract: Rational structure and content of CAD system for the planetary gear transmission are discussed from the point of design method．The model of optimum design is introduced．The design objective of minimizing volume and module simultaneously are adopted‚and a graded optimum strategy is presented．Different objective functions were used for the first time in accordance with different variables．As a result efficiency of calculation is raised‚optimization of an overall situation and reliability of an optimum solution are ensured．Besides‚the control methods of the program are put forward as a requirement of actual design． 

Key words: planetary gear transmission, computer aided design, optimum design