Nowadays With increasing concern about global warming and shortage of fossil fuels, application of renewable energy resources has become more important. Photovoltaic (PV) is predicted to have the largest generation, up to 60% of the total energy by the end of this century, because the light of sun is free and available almost everywhere. Grid connected inverters are the basic units of PV system which act as an interface between PV module and grid. Since the output voltage of PV modules is not high enough, a boost type DC-DC converter is used between PV module and inverter. The added converter increases inverter cost and degrades efficiency. So the current trend is toward single-stage inverters. Whereas single-stage transformerless inverters (SSTI) are more efficient, less expensive and have lower size and weight in comparison to isolated inverters, they are an attractive solution for connecting low-voltage single phase PV systems to the grid. Beside the excellences of SSTI, the leakage current issue which depends on structure and control of inverter should be considered in designing these inverters. In this thesis, three new low power inverters for PV application are proposed. The introduced inverters have single-stage structure and the isolation between PV module and grid is omitted in order to reduce size and weight and increase the overall efficiency. It is tried to improve inverter features including voltage gain and efficiency as well as producing high quality output voltage. Although these structures include four active switches, they have only one or two high frequency active switches. Reducing the number of high frequency active switches beside providing soft switching condition for semiconductor devices, reduces gate drive and switching losses and decreases inverter size by means of increasing frequency. Another feature of introduced inverters is simplicity of structure and control technique which leads to manufacturing cost reduction and reliability increment. The leakage current problem is considered in proposed structures and it is totally omitted in two of the three introduced structures. The proposed circuits are completely analyzed and the design principle and recommended control technique of each structure is presented. To confirm the validity of theoretical analysis, a prototype of each inverter with output power lower than 100w is simulated and tested in the lab. Keywords : Single-stage Inverter, Transformerless Inverter, Leakage Current.