Background Occlusal splints have been the preferred modalities in the management of nnyofascial tennporomandibular disorders (TMDs), but now controversy exists in reporting whether they are successful for TMDs treatments. The aim of this study was to give objective evidence to the assessment of treatment effect of occlusal splints for myofascial TMDs patients by clinical assessments and surface electromyography (sEMG) measurements of masseter muscles (MM). Methods Thirty-six patients (12 males and 24 females) aged 16-57 (38 11) years participated in the study. All participants diagnosed with myofascial TMD were randomized into two groups (18 of each). Patients in the first group (A) were treated with occlusal splints for 1 month, while patients in the second group (B) were treated with placebo (non-occluding palatal) splints. Clinical assessments were performed at the beginning of the study and 1 month after treatment. sEMG measurements for MM were performed at mandibular postural position (MPP) and maximum intercuspal contacted position (ICP) 1 month after the treatment. The root mean square (RMS) and the median frequency (MF) as linear indices of sEMG data were used to demonstrate muscle activity and muscle fatigue. Data were analyzed by ANOVA and post hoc SNK test. The differences were considered significant at P<0.05. Results It was found that 89% of group A either completely recovered (39%) or clinically improved (50%), while only 22% of group B had a spontaneous improvement. sEMG analysis showed that at MPP, the mean of RMS value of MM in group A was lower than that of group B, which shows statistical differences (P<0.01). At ICP, the RMS value of MM in group A was higher than that of group B, which shows statistical differences (P<0.01). At MPP, MF value of MM in group A was higher than that of group B (P<0.05). At ICP, MF value of MM was lower than that of group B (P<0.01). Conclusions Occlusal splint could eliminate or improve the signs and symptoms of TMD patients with myofascial pain. sEMG analysis indicates that the wearing of occlusal splints may reduce the degree of fatigue of the masticatory muscles. The splint therapy outcome has a correlation with the electromyographic changes in the masticatory muscles.