Synchronous belt drives are often used in bidirectional applications such as priniting machines, high-performance machine tools and industrial robotics, and are required to provide high transmission and positioning accuracy. Helical synchronous belt drives, in comparison to conventional synchronous belt drives, are effective for reducing noise and transmission error over single pitches of the pulley. In the present study, the transmission error of a helical synchronous belt drive under installation tension was investigated both theoretically and experimentally for the case when the drive reverses direction. The computed transmission error agrees with the experimental results, confirming the applicability of the proposed theoretical analysis for transmission error. It is found that transmission error in this case is generated by a change in the contact state between the belt tooth flank and the pulley tooth flank, and axial displacement of the belt between the driving pulley and the driven pulley. By setting the installation tension as high as possible, the transmission error can be reduced and stabilized more quickly. In addition, when the outside diameters of both the driving and driven pulleys are equal, transmission error does not occur.