In practical applications, the effect of the temperature changing rate on the thermal drift error of a fiber optic gyroscope (FOG) can not be ignored fiber optic gyroscope (FOG) due to temperature variation. Effect of temperature variation on thermal drift error of FOG in theory is analyzed. Then a temperature cycling experiment is designed to obtain the thermal drift error in seven temperature rates. Based on experimental data, a function is utilized to describe the relationship between temperature changing rate and thermal drift error of the FOG. With this fitting function, the thermal drift error of a FOG under any temperature changing rate will be obtained, and parameters of the function can be used to evaluate the thermal performance of a FOG.