As one process of developing the Rotary engine with low fuel consumption，we made an experiment on the air-fuel ratio. And we tried to seek the air-fuel ratio for good fuel efficiency，for little dropping in torque and for good driveability without misfire at full load. The main results are as follows. For the fuel of Rotary engine，L.P.G. can be used in leaner air-fuel ratio than Gasoline. And Rotary engine using L.P.G. for fuel have better fuel consumption than using Gasoline. Especially， the consumption is best at low speed 2000～3000 r.p.m., and so L.P.G. is suitable fuel for Rotary engine at low speed.