This article reports the study of rate-dependent mechanical properties of thermal-oxidized PMR-15 polyimide resins with a nanoindenter. A series of PMR-15 resin specimens have been isothermally aged at various temperatures, times, and pressures. The strain rate sensitivity of oxidized surface layer obtained at each aging condition has been determined from nanoindentation creep experiments using constant displacement-rate (h) method. Results show that the average strain rate sensitivity in the oxidized surface layer is notably higher than that in the unoxidized interior, indicating that the oxidized surface layer has limited ductility and thus is susceptible to fracture. Effects of aging environments (time, temperature, and pressure) on mechanical properties are also examined. After passing the initial oxidation stage, the change in strain rate sensitivity become insignificant and less sensitive to aging parameters.