Masticatory muscle activity is coordinated with perioral muscle during chewing. Subjects with competent lips usually chew with the lips in light contact, whereas subjects with incompetent lips possibly have dysfunctional chewing. In this study, the electromyographic (EMG) activities of the lower lip and masseter muscles were recorded when chewing with the lips in contact and apart. At first, 37 subjects were divided into an incompetent lip group and competent lip group on the basis of EMG activity of the lower lip muscle at rest. The durations of the masseter nonactive phase and total phase when chewing with lips in contact were shorter in the incompetent lip group than in the competent lip group. In the incompetent lip group, when chewing with the lips apart, the EMG activity of lower lip in the masseter nonactive phase was significantly (P < .05) higher than in the competent lip group, but there was no difference in the EMG activity in the masseter active phase between two groups. Our results suggest that subjects with incompetent lips have difficulty chewing while their lips are relaxed. We conclude that the inability of sealing the lips and lip dysfunction could possibly affect masticatory function.