A simple wavelength-independent scale factor model is established for a closed-loop interferometer fiber optic gyroscope (IFOG) and a method to keep the scale factor radiation tolerant and temperature stable in a high performance IFOG for space application is proposed. The half-wave voltage ($V_{\pi }$) of the multifunction gyro chip at different wavelengths and temperatures is measured and the radiation-independent inherent parameter of the modulator is picked out and found be proportional to the temperature and slightly wavelength dependent. An experimental IFOG is developed and the scale factor is measured at different temperatures and under ${}^{60}\mathrm{Co}$ irradiation, respectively. Less than a 10 ppm scale factor instability is achieved within the $40\mathrm{°C}$ to $+60\mathrm{°C}$ temperature range and more than 100 krad(Si) total radiation dose.