Helical synchronous belts have been confirmed to reduce transmission error and noise. However, side tracking in the axial direction occurs in helical synchronous belts under no load. When torque acts on the belt, in addition to incurring side tracking, offset, which is a difference in the axial direction between the belt on the driving pulley and the belt on the driven pulley, is occurred. It seems that side tracking and offset result in damage at the side face of the belt because the belt is forced against the pulley flange. Therefore, it is necessary that it clarifies the process in which side tracking and offset grow and investigates methods for reducing side tracking and offset. In the present study, side tracking and offset in a helical synchronous belt were investigated both theoretically and experimentally under torque. The experimental results were agreed closely with the calculated values. Side tracking was confirmed to occur because of shearing strains of the belt at the beginning of meshing and offset, and could be reduced by lowering the installation tension. Offset showed to increase proportionately with torque.