The flatness of on-resistance is an important performance of the power device when it is used as a switching device. The factors which have considerable influences on the flatness of on-resistance have been studied and optimized to improve the performances of the device. The specific channel resistance is a major part of on-resistance for the UMOS with a low breakdown voltage. Therefore, the computed equation of the channel resistance was given and analyzed in this paper. The trends of the on-resistance flatness were investigated with the P-base implant dose and the gate oxide thickness using Sentaurus TCAD simulation. The simulation results showed that a better flatness of on-resistance could be obtained by reducing the gate oxide thickness and the implant dose of P-base region.