This thesis presents experimental and modeling studies of creep performance of modified 9Cr-1Mo steel with and without an oxidation-resistant coating. A deformation-mechanism-based true-stress (DMTS) creep model is proposed for predicting creep rate and creep life of modified 9Cr-1Mo steels. The DMTS model considers three well-recognized deformation mechanisms:​ dislocation glide, dislocation climb, and grain boundary sliding. Constant-load creep testing is conducted on modified 9Cr-1Mo steel in forged form, F91 (9Cr-1Mo-V-Nb). The creep performance of three types of F91 coupons, pristine, coated, and aged coupons is studied. The creep data obtained in the present study and those reported from the literature are used to validate the DMTS model. Four major achievements are obtained from this study. (i) The creep rate behavior of nine modified 9Cr-1Mo steels in various product forms including tubes, pipes, plates, and forging is systematically characterized in terms of power-laws representing the above-identified deformation mechanisms;​ (ii) The creep strain-time behavior of the pristine and coated F91 steel is analyzed to verify the basic DMTS model and the modified DMTS model with consideration of oxidation, showing excellent agreement with the experimental observations;​ (iii) The effect of microstructural evolution is studied using aged F91 coupons, showing that Lave phase formation associated with grain boundary sliding is mostly responsible for the increased creep rate and shortening of creep life as compared to the pristine material;​ (iv) Last but not the least, long-term creep lives (>​104 hours) of modified Grade 91steels are predicted using the modified DMTS model, showing pronounced improvement over the basic model, indicating that oxidation effect is significant in long-time creep, whereas direct extrapolation from short-term creep test data is not correct for modified 9Cr-1Mo steels, because short-term creep life does not include enough oxidation effect.