This research investigated spray characteristics using LPG fuel under compression ignition to contribute to develop a high efficiency LPG fuel is an environmentally-friendly fuel since it emits lower $CO_2$ compare to other conventional fuels. In order to observe spray process, a high speed digital camera and high pressure common-rail injector were applied. Using the spray behaviors of LPG and diesel fuel from the experiment, this research analyzed the mixing process of air-fuel mixture numerically with FLUENT 6.3 when LPG and diesel fuel injected directly into the cylinder while compression stroke occurs. Spray characteristics of LPG fuel was investigated by using numerical method, in which KH-RT model was adapted for phase change. As a result of numerical analysis, this work found out that LPG spray has a wider mixing formation and uniform diffusion of air-fuel mixture compare to diesel.