To study the effect of microgravity on peripheral oxygen saturation (SpO2) in rats, tail-suspended rats were applied to simulate microgravity environment. SpO2 and arterial oxygen saturation (SaO2) were measured by pulse oximeter and arterial blood gas analyzer (ABGA) respectively on the 14th day, 21st day and 28th day in tail-suspended group and control group. Paired t-test shows that SpO2 was significantly lower than SaO2 in tail-suspended group on the 14th day (P < 0.05), the 21st day ( P < 0.05) and the 28th day ( P < 0.01). The ANOVA results shows that modeling time had significant effect on SpO 2 value but no effect on SaO2 value in tail-suspended group. These results indicate that pulse oximeter may be not suitable for oxygen saturation test in microgravity environment.