Multi-core processors are becoming widely deployed. However, more cores consume more power. A power saving technique for multi-core systems based on the observation of critical sections is proposed. Since only one thread works in a critical section, other threads on other cores that access the same resource would be busy waiting. The proposed power-saving technique leads to the energy savings (11 to 15 percent) and the lowest values of Energy Delay Product as compared with the other power-saving techniques.