An oceanic general circulation model is adopted to study the oceanic response to atmospheric forcing and the formation mechanism of the North Pacific Gyre Oscillation (NPGO). The control experiment shows that the numerical model can successfully simulate the spatio-temporal features of the NPGO, suggesting that the model can be used to study the formation mechanism of the NPGO. The SST anomaly fields simulated by the model being forced with composed atmospheric fields of positive strong NPGO years are similar to the spatial pattern of the NPGO mode, suggesting that the atmospheric forcing is the direct mechanism of NPGO fromation. Sensitivity experiments reveal that the dynamic effect of wind is most important to the formation of NPGO, especially the effect of zonal wind stresses.