The optimal speed ratio of a series-parallel hybrid transmission was selected using the design of experiment （DOE） method and the effect of transmission gears on vehicle performance was investigated for the transmission matching of series-parallel hybrid vehicles. The vehicle simulation model and control strategy were developed and verified by bench tests， and the vehicle acceleration performance， climbing performance and fuel consumption with three types of transmission were analyzed. The results show that more transmission gears can improve the dynamics performance of the vehicle and reduce the demand for battery discharge power and electricity. However， the improvement in the vehicle fuel consumption is insignificant. Further simulations indicate that the number of transmission speed does not have a significant impact on the vehicle fuel consumption even when the fuel consumption of engines is improved at the low-speed， mid-to-high load ranges.