Abstract: The error composite modeling and compensating of temperature effect and scale factor nonlinearity of fiber optic gyroscope(FOG) was studied. The mechanism of thermal affect arising from nonreciprocity of optic-loop was analyzed. Based on the severe nonlinearity drift error induced by temperature change and scale factor nonlinearity, an practical and precise composite model was proposed to compensate the error caused by temperature and scale factor-s nonlinearity. The test experiment and data processing were carried out at full temperatures and full rotate rates of FOG-VG951 and indicate, utilizing piece-wise modeling and compensating the FOG, its drift error can be minished from 4.4 (°)/s to 0.1 (°)/s , which is much better than the conventional modeling and compensating means, and the experimental results indicate that the method is useful and feasible.