We present a study of electric field effect on the efficiency of GaN/In0.1Ga0.9N p-i-n solar cells by using the advanced physical models of semiconductor devices (APSYS) simulation program. In this study, the electric field strength and other parameters such as optimum thickness of p-type layer and efficiency of GaN/In0.1Ga0.9N p-i-n solar cells with different i-layer thicknesses have been performed. On the basis of simulating results, for a high efficiency solar cell, it is found that the optimum p-type layer concentration is above 4 x 1016 cm-3 and the suitable thickness is between 0.1 to 0.2 μm. For different i-layer thickness and p-doping concentrations, a critical electric field (Fc) has been found at 100 kV/cm. It is worth to note that when the electric field strength of i-layer below Fc value, the solar cell efficiency will dramatically decrease. Thus Fc can be seen as an index for acquiring the quality of solar device.