As the energy crisis and the pollution are serious, the exploitation of solar has received more and more attentions. It is well known that for a given solar radiation intensity and solar cell temperature there exists a maximum power point at which the power generated from the PV panel is at its maximum. In order to improve the efficiency of the system, the main method is to regulate the output of array to develop the maximum power point tracking (MPPT). In this paper the principle and control method of DC/DC conversion in grid-connected photovoltaic system are experimentally discussed. The conductance incremental method is analyzed in detail, and an improved variable step-size control method is implemented for MPPT with pulse width modulation. The experimental results prove the feasibility and correctness of the control method.