A novel transformerless boost inverter for stand-alone photovoltaic generation systems is proposed in this paper. The proposed inverter combines the boost converter with the traditional bridge inverter. The switch S1 not only realizes the boost function but also participates in inverting process. The inverter has a higher voltage gain and good characteristics when the inductor L1 is operated in discontinuous mode(DCM) and the nonpolarized capacitor can be chosen as bus capacitor, which makes the volume smaller and the service life of the inverter is increased. The inverter consists of five switches in which only two switches are operated at high frequency state and a monopole sinusoidal pulse width modulation(SPWM) strategy is used. Therefore, the modulation strategy of switches is very simple and the switching loss is reduced. The principle of inverter is described in detail and mathematical models are built by small-signal analysis. Finally, the correctness of the theoretical analysis is verified by simulation and experiment.