Stair climbing is an intense physical activity and requires large range of motion at the joints, adequate muscle strength, and balance control. A powered stairmill, integrated with a gait rehabilitation device, can potentially be used for training those who have difficulty climbing stairs. In order to assess the effectiveness of such an approach, it is necessary to understand the similarities and differences in walking on regular stairs and on a stairmill. We conducted an experiment to compare the differences in kinematics and muscle activations during climbing on regular stairs and a stairmill. Twelve subjects participated in this study. They first walked on regular stairs five times and then performed a one-minute continuous walking on a stairmill. The results showed several important differences. During continuous walking on a stairmill, when compared to regular stairs, there was (i) an increase in the percentage of stance phase during a walking cycle, (ii) a higher angle of plantarflexion of the ankle during the transition from stance phase to swing phase, and (iii) a decrease in muscle activation of the tibialis anterior during swing phase. These differences would provide additional insights into the design of future rehabilitation systems and to interpret human data obtained from stairmills.