The present studies adopted the theoretical framework of activity- and purpose-related incentives (Rheinberg, 2008) to explain the maintenance of physical activity. We hypothesized that activity-related incentives (e.g., “fun”) increase more than purpose-related incentives (e.g., “health”) between the initiation and maintenance phase of physical activity. Additionally, change in activity-related incentives was hypothesized to be a better predictor of maintenance of physical activity than change in purpose-related incentives. Two correlative field studies with rehabilitation patients (Study 1) and Nordic Walkers (Study 2) were conducted to test the hypotheses. Participants’ incentives of physical activity were measured at the beginning of exercising and two weeks (Study 1; T2) and three months (Study 2; T2) later. At T2, participants were asked for their current physical activity. Both studies showed a greater change of activity-related incentives than purpose-related incentives. Furthermore, change in activity-related incentives was more predictive of the maintenance of physical activity than change in purpose-related incentives. The results showed the important role of activity-related incentives in maintenance of physical activity. The theoretical contribution to physical activity maintenance research and practical implications for health promotion programs were discussed.