In a bi-fuel engine using gasoline and LPG fuel, with the current ignition timing for gasoline being used, the effective performance could not be taken in LPG fuel supply mode. The ignition timing in LPG fuel mode must be advanced much more than that of gasoline mode for the compensation of its lower flame speed, due to engine torque drop. This study aims to develop the control system for ignition spark timing conversion which is composed of hardwares and control algorithm for gasoline/LPG engine. We propose the control system which can advance the ignition spark timing in LPG fuel mode more than used in gasoline fuel mode. The advance of ignition timing is achieved by change of the ignition dwell time of coil igniter. The engine torque and F/E(Fuel-Economy) in LPG fuel mode are measured to evaluate the difference of engine performance between before and alter changing ignition spark timings. The engine torque and F/E are increased respectively, which proves the developed control system is effective so much for gasoline and LPG bi-fuel engine.