This paper introduces the development of transmission design expert system base on MATLAB. It is discussed in detail that the establishment of expert system knowledge base， input and output (I/O) interface， material database and parameter database. In the part of the knowledge base, such as modeling of transmission design calculation, building of material database and parameter database are expounded detailedly. The I/O interface is built with MATLAB “Graphical User Interface (GUI)” technology, which is good at organizing knowledge, so ensure the consistency and rationality between I/O interface and knowledge base. Furthermore, the I/O interface makes the communication between the subsystems more convenience. Finally, with the help of expert system, a (4 + 1) gears Manual Transmission is designed, The result shows that the transmission expert system can shorten automobile transmission design calculating time and improve the calculation precision.