Nano-scale power splitter based on Si plasmonic waveguides are designed by utilizing the multimode interference (MMI) coupler. Effective dielectric method and longitudinal modal transmission-line theory are used for simulating the light propagation and optimizing the structural parameters at 3-D guiding geometry. The designed $1{ imes}2$ 50:50 MMI power splitter has a nano-scale size of only $800nm{ imes}850nm$. In order to achieve a variable power splitting ratio, a $2{ imes}2$ MMI coupler is designed and the corresponding power splitting ratio can be tuned in the range of 78.5%:15.5%~5.5%:86.6%. Also, it is shown that it has a large bandwidth of $1.5{mu}m{sim}1.7{mu}m$. In this range, the transmission is over 0.8.