Planetary gear trains have the advantages of compactness, light weight, high speed reduction and differential drive, and can provide several speed ratios by means of selecting different clutches and brakes. They are widely used in the automatic transmissions for automobiles. The purpose of this work is to develop a systematic methodology for the design of automatic transmissions for automobiles. First, the specification is established by using concept of fundamental geared entities. Second, a methodology of structure synthesis is developed for establishing gear trains graphs table with up to three fundamental geared entities. Third, a method for systematically enumerating all possible clutching sequences is established and a method for position of clutches and brakes is developed to establish automatic transmissions graphs table. Forth, a procedure for the selecting of a most efficient clutching sequence associated with a automatic transmission is developed. Finally, the systematic theory integrates preceding methodologies and a computer software package is to developed. The computer software package provides user a powerful tool to design automatic transmissions for automobiles.