Using helical teeth for synchronous belt drives is very effective in reducing noise. However, it is necessary to find out how durable such belts are. In this study, running fatigue tests on both helical and conventional synchronous belts, used generally in industry and in vehicle engines, were carried out under a constant transmission force. The helix angles of the helical belts were 5°and l0°. Other dimensions were the same as those of conventional belts. The life of the helical belts was compared with that of the conventional belts. It was found that the belt life was almost the same for each type and was longest for both when installation tension set up as the slack side tension for the transmission force was the lowest. The results proved that when noise is a crucial factor, helical belts should be used.