A transmission system for a straddle vehicle which provides two operating modes, namely semi-automatic and automatic. With the semi-automatic transmission system, an operator only uses a switching means mounted on a steering bar of the vehicle for activating a clutch actuator and a shift actuator. The clutch actuator disengages a transmission clutch while the shift actuator operates a shifter for changing the transmission ratio. With the automatic transmission system, an Electronic Control Unit reads inputs signals such as speed of the engine (RPM), speed of the vehicle, opening of a throttle valve and position of a shifter and accomplishes a smooth shifting by activating a clutch actuator, a shift actuator and a modulated controlling valve.