To design an all-terrain vehicle, it is required that the transmission system should withstand all the hurdles on the terrain. The terrain includes the mud pool, rock crawl, dunes etc. The main aim of the transmission system is to provide required torque and power that is being generated by the engine to the wheels as per cockpit sitting driver requirement. This includes economically simplifying the design of transmission system without compromising the performance and safety standards. This motion comprises of material selection, material procurement, gearbox designing, gearbox manufacturing, Finite Element Analysis (FEA) and simulation test to check for the failure. Since All-terrain vehicle is prone to uneven road conditions and harsh environment due to which a concept of constant and continuous power transmission should be embedded in its design. For which a continuous variable transmission (CVT)i.e. pulley and belt system and designed gearbox work hand in hand to obtain the desired reduction ratio. For designing a transmission system tractive effort, Gradability, vehicle resistance, and vehicle maximum speed is to be calculated. With the above-calculated data, the required reduction ratio is calculated. Finite Element Analysis(FEA) is done for the validation of the design.