A new helical synchronous belt was developed for the purpose of noise reduction. Dimensions of the helical belts are based on the 8 mm pitch curvilinear synchronous belt, called YU, for automotive engines. The helix angles of the helical belts are 3°, 5°and 10°. In this study, the relationship between the noise and the helix angle of the helical synchronous belts is discussed. The noise level of the helical synchronous belts with helix angle of 5° and 10° is lower by around 5 dB (A)than that of the conventional synchronous belt. In the case of no load, the noise level of the helical belt with helix angle 10° is lower by about 10 dB (A) than that of the conventional belt.