Abstract: A fuel injection control system accurately determines the amount of fuel to be injected by compensating the measured crankcase pressure in view of various conditions including intake air condition, effects of exhaust gas pressure by other cylinder and others, with use of one pressure sensor. The fuel injection control system includes a pressure sensor installed in a cylinder of the engine for detecting crankcase pressure, a fuel injector for injecting fuel to the engine on the basis of the crankcase pressure detected by the pressure sensor, and a device for compensating the amount of fuel injected from the fuel injector depending on the condition of intake air in the crankcase. Another aspect of the system additionally includes controlling timing for detecting the crankcase pressure by the pressure sensor depending on the rotation rate of the engine.