Belt side tracking caused by the helix angle of the tooth trace of a helical synchronous belt under no load was investigated in the present study. Belt side tracking is an important problem because it influences the transmission characteristics, belt life and noise characteristics. Experimental results of belt side tracking were correlated the force on the belt tooth when the belt tooth began to mesh with the pulley tooth. It was confirmed that the amount of belt side tracking is proportional to the magnitude of the installation tension and the helix angle, and is inversely proportional to the belt width.