The load in power system usually can be fluctuated, sometimes awfully, due to sudden disturbance, measuring and forecasting error, or even its inherent nature. Therefore, the conventional deterministic generation dispatch problems are not enough to operate power system. To deal with such uncertainties, Gaussian distribution is adopted and then probabilistic optimal power flow (OPF) problem is formulated in this paper. Some case studies are presented for the simulation assumed that load is not deterministic anymore. The results of the case studies present how the undetermined loads affect generation cost.