Abstract: Previous studies have shown that the ventral part of prefrontal cortex (vPFC) plays an important role in drug addiction. The increased interest in γ oscillations, now widely regarded as functionally relevant signals of the brain, underlines the importance of the concept of event-related oscillations for bridging the gap between single neurons and neural assemblies. In the present study, we examined the dynamics of γ oscillation activity (or γ-band power) associated with the development of morphine dependence in the vPFC by using electrophysiological techniques after injecting morphine (0, 0.5, 1.6, 5.0 and 8.0 mg/kg) in rhesus monkeys. The results show that the γ-band power had a long-term decrease associated with the development of morphine dependence, and the decrease in γ power had a negative relationship with the concentration of morphine. Those results suggest strongly that γ oscillation activities in the vPFC reflect morphine addiction, and indicate that the EEG γ oscillation may reflect the processes underlying morphine addiction.