This study was performed to investigate the effects of changes of head posture and cervical spine shape on the mandibular resting or clenching electromyographic(EMG) activity in anterior temporalis(TA), masseter(MM), sternocleidomastoid muscle(SCM) and trapezius insertion(TI). 30 patients with Temporomandibular Disorders(TMDs) participated in this study. EMG activity($\mu$V) at rest and clenching was observed in four head postures, namely natural head posture(NHP), forward head posture(FHP), upward head posture(UHP), and downward head posture(DHP). For taking in upward or downward head posture head was inclined 10$^{\circ}$ upward or downward and CROM$^\textregistered$(cervical-range-of motion, Performance attainment Inc., USA) was used to maintain same posture during the procedure, and BioEMG$^\textregistered$ (Bioelectromyograph, Bioresearch Inc., USA) was used to record EMG activity in the above four muscles at eight locations on both sides. The recorded EMG activity($\mu\textrm{V}$) were compared and analyzed by cervical spine shape such as the head position from plum line, cervical curvature, and cervical inclination. Head position from plum line was measured in vertical plate calibrated with cm scale, comical curvature by radius was measured with adjustable curved ruler, and cervical inclination by cervical vertebrae tangent(CVT)was measured in lateral cephalograph. The results obtained were as follows : 1. Mean value of head position from plum line, cervical curvature, and cervical inclination were 4.8cm, 26.7cm, and 86.6$^{\circ}$, respectively, And There were no correlationship among these items. 2. For resting EMG activity by head posture, the value in anterior temporalis was higher at FHP than at DHP, the value in masseter was higher at FHP than at NHP, and DHP, the value in sternocleidomastoid muscle was higher at UHP than at NHP, and the value in trapezius insertion was higher at FHP and DHP than, NHP and UHP. The clenching EMG activity, however, did not show any difference by head posture. 3. Comparison of resting and clenching EMG activity between higher and lower groups by head position from plum line, cervical curvature, and cervical inclination did not show any significant difference. From this result, the author concluded that the cervical spine shape had not significantly affected to EMG activity in usual patients with TMDs.