A photovoltaic system (PV) connected to the utility grid through three-phase voltage source inverter (VSI) has been controlled in this paper for achieving maximum power point tracking (MPPT). In this paper, a modified perturb and observe (P&O) technique has been proposed for achieving MPPT of PV system using DC-DC boost converter. The proposed algorithm is used to obtain better capture of energy with minimum steady state oscillations. In this technique, the duty cycle is determined using a variable step-size to achieve MPPT accurately. A digital simulation using Matlab/Simulink package is carried out  to demonstrate  the advantage of the proposed system performance compared to the conventional technique under varying irradiation and temperature levels. The obtained  results investigate  that the modified algorithm can track the maximum power point (MPP) accurately besides, it extracts the additional power during the variation in weather conditions as compared to the conventional P&O algorithm.