Helical timing belts have been developed in order to reduce the noise that occurs when conventional timing belts are driven. Helical timing belts are characterized by synchronous rotation. Although several studies have been performed to clarify the noise characteristics and belt life of heling timing belts, the transmission error of those belts remains unclear. In the present study, the transmission error having a period of one pitch of the pulley was investigated both theoretically and experimentally for helical timing belt drives. Experimental conditions were such that the transmission force acts on the helical timing belts under quasi-static conditions and the belt incurs belt climbing at the beginning of meshing and at the end of mashing. Experimental results obtained for the transmission error agreed closely with the computed results. The computed results revealed that helical timing belts can be analyzed as asset of very narrow belts for which the helix angle is zero. The transmission error was found to decrease when the helix angle or the belt width increase within arrange defined such that the face advance is less than one belt pitch. In addition, there exists an appropriate installation tension that reduces the transmission error.