Abstract This paper studies the influences of different fuel injection strategies on flame propagation and combustion characteristics in a glow plug assisted direct-injection natural gas engine. Our previously existing version of the KIVA-3V CFD code incorporates a model of internal flows in the gaseous fuel injector and also a detailed model of the glow plug and its shield. This study was conducted by using the KIVA-3V code with added improved emissions models, including a detailed kinetic chemical model and a modified phenomenological soot model. In the simulation, a low temperature natural gas mechanism is selected to estimate the gaseous species, and acetylene was chosen as the key species to model the soot formation. The simulation indicated that the fuel injection duration, injector nozzle size and injection angle can affect the natural gas combustion characteristics. Generally, shorter injection duration reduces both peak cylinder pressure and emissions in the natural gas engine; however, the natural gas flame propagation cannot be maintained once the injected fuel mass is lower than a limit. The simulation also reveals that the injector nozzle size highly affects the natural gas combustion by inducing diffusion combustion for a small diameter nozzle and partially premixed combustion for a large diameter nozzle. Compared to a large nozzle, a small nozzle results in a faster pressure rise with more engine emissions and lower combustion efficiency. The fuel injection angle can influence natural gas combustion characteristics by affecting the flame propagation out of the glow plug shield in the initial combustion stage.