Abstract: Objective To study the effect of chronic unpredictable stress on ultra-structures of masseter in rats. Methods Thirty-two male Wistar rats,at the age of 12 weeks,were randomly divided into experimental group and control group. Each group was divided into 3-week subgroup and 6-week subgroup（8 rats in each group）. A model of rats with chronic unpredictable stress was established for the experimental group. Rats in the 3-and 6-week subgroups were killed after 3 and 6 weeks,respectively. Some of the masseter and internal pterygoid muscles of rats were cut,and changes in ultra-structures of masseter muscle were observed under transmission electron microscope. Results Under transmission electron microscope,ultra-structures of masseter muscle cells changed significantly in the experimental group. The myofilament was deranged,the "z-line" was warped,and mitochondria with loss and degrade of cristae were found in 3-week subgroup. Severe vacuolus changes in mitochondria were observed in 6-week subgroup. No obvious change was found in the control group. Conclusion Chronic emotional chronic unpredictable stress induces injury of masseter in rats at ultra-structure level.