The importance of pollutant control and shortage of fossil fuel reservoirs have caused the development of injector systems and researches on optimum fuel consumption of internal combustion engines. The main purpose of this work is to study the effect of fuel injection start angle on engine performance features such as indicated power, exhaust emissions (CO and HC), ignition delay and fast burn length in a single cylinder port injection SI engine using gasoline and natural gas individually as fuel. Injection period, ignition timing, engine speed and throttle plate position were fixed and start angle of injection (SOI) was varied. The obtained results show that higher indicated power and lower CO emission are achieved when SOI is adjusted so that the injecting fuel and flowing air are entering simultaneously into the cylinder; however, higher unburned HC emission is resulted at the condition. Heat release rate analysis was used to evaluate ignition delay and fast burn length. The results show that the lowest ignition delay happens when the SOI is adjusted so that the part of injected fuel at the late intake stroke is higher; and the fast burn length is decreased as both injecting fuel and flowing air are entering into the cylinder during the injection period.