Nowadays, motorbikes are still the main and most popular transport in Asian countries, especially, in Vietnam. However, the manifold intake systems in motorbikes are usually designed in a simple structure, which can reduce engine performance and increases fuel consumption in motorbikes. This paper presents the study of the improvement of the intake system in a 125cc motorbike engine to enhance air-fuel mixing quality that can increase the engine performance. The study employs the ANSYS software to obtain an optimal intake systemthrough the tumble and swirl ratios in different simulation cases. Another model wasalso built by Matlab/Simulink to examine the effects of swirl and tumble ratios on engine performance and fuel consumption at various engine speeds. The simulation results show that the modified intake system withan incline angle of 30o has optimal tumble and swirl ratios. Accordingly in conclusion, power output and torque of the engine with the modified intake system are higher, while the fuel consumption is lower than previouslyfound.