The primary objective of this study was to investigate the effect of the postural change between a normal seating posture and a reclined posture in vehicles by using X-ray to measure individual variabilities of skeletal alignments in a reclined seat. Another objective was to understand the effects of this postural change on injury outcomes. Therefore, human body models representing those individual variabilities were constructed by modifying THUMS ver.4, and occupant kinematics in frontal crashes were analyzed. Three target seating postures were defined, at 24, 35 and 45 degrees. Based on the analysis results of the X-ray data, the lumbar and pelvic alignments of THUMS were modified. Three different reclining angles and skeletal alignment types were compared. At 45 degrees, the forward displacement of whole body and the downward movement of the pelvis were observed. Additionally, the differences in kinematics of the upper body by the different alignment types was also confirmed. Furthermore, more head acceleration, chest rib fractures and lumbar spine stress were observed, as well as a higher risk of submarining. It was found that a reclined posture could significantly affect occupant kinematics and injury outcomes in a collision.