The purpose of this research is to perform modeling and control of a non-linear system for the entire LPG engine including the electronic control throttle servo system. During the non-liner system modeling, we propose the sliding mode control method using non-linear VSS observer to control engine speed. Finally, we confirm the validity of the analysis by the sliding mode control method, which is most systematized among non-linear control theories, in the system that is designed to control those elements having non-linear properties like the one in this study.