This paper presents a study on the dynamic behavior of helical gears of full depth and long teeth with various standard pressure and helix angles. Natural frequencies of flexural vibration of gear body were measured and compared with the results calculated by Mindlin's method. The relations between these natural frequencies and spectra of circumferential, radial, and axial vibrations were investigated. The circumferential, radial, and axial vibration accelerations, and the root stresses were measured under different running conditions by using a gear testing machine of the power absorbing type. On the basis of these results, the effects of pressure angle, helix angle, and whole tooth depth on the vibrations and dynamic load of helical gears were clarified to a considerable extent.