To eliminate the common-mode leakage current in the transformer less photovoltaic gridconnected system, an improved single-phase inverter topology is presented. The improved transformer less inverter can sustain the same low input voltage as the full-bridge inverter and guarantee to completely meet the condition of eliminating common-mode leakage current. Both the unipolar sinusoidal pulse width modulation (SPWM) as well as the double frequency SPWM control strategy can be applied to implement the three-level output in the presented inverter. The high efficiency and convenient thermal design are achieved thanks to the decoupling of two additional switches connected to the dc side. Moreover, the higher frequency and lower current ripples are obtained by adopting the double-frequency SPWM, and thus the total harmonic distortion of the grid-connected current are reduced greatly. Furthermore, the influence of the phase shift between the output voltage and current, and the influence of the junction capacitances of the power switches are analyzed in detail. Finally, a 1-kW prototype has been simulated and tested to verify the theoretical analysis of this paper.