A subwavelength plasmonic laser structure based on a metal-dielectric-metal nanocavity is proposed and numerically simulated by using the finite difference time domain method with perfectly matched layer absorbing boundary condition. The nanocavity model and gain analysis are respectively given. The simulation results show that the losses within the nanocavity (including surface plasmon losses) can be compensated by the gain material and the threshold gain of the laser is about 1.5 × 103 cm−1 with the peak wavelength around 1,550 nm. The new device would be an important step toward a fully integrated surface plasmon circuits.