Effects of an activator on the temporal and masseter muscles activities were investigated in subjects with an Angle Class II division 1 malocclusions in this study. The measurements were done on 21 subjects. Twelve of these subjects were in the treatment group and 9 were in the control group. Average ages of treatment group was 11.42±0.29 years and average ages of control group was 10.68±0.52 years. Muscle activities were investigated at the beginning and at the end of this research, both with and without the activator. The increase of activities in the rest position and decrease of activities in maximum biting of both muscles with activator were found to be statistically significant at the beginning of the treatment. Activities of both muscles with the activator decreased in the rest position at the end of treatment when compared to the beginning of the treatment. No change has been observed between the activities of masseter and temporal muscles recorded at the beginning and at the end of this investigation without the activator.