In order to study a disintegration mechanism of a high viscous liquid jet issuing from a cylindrical nozzle, a break-up length of the jet was measured. The main parameters in experiments were liquid viscosity, injection speed, diameter and length of a nozzle, and shape of the nozzle entrance. When the viscosity of the liquid was changed widely, the break-up lengths measured by an electrical resistance method were not uniquely expressed with the Reynolds number in the nozzle flows. The smooth jet appared only in low Reynolds number conditions, if the kinematic viscosity increased up to 20×10-6m2/s. However, the high viscosity liquid jet in the wavy flow region showed a smooth surface at the nozzle exit and development of the surface wave suddenly occurred at far away from the nozzle exit. This late development of the wavy flow was predominating on the jet issuing through a long nozzle or a nozzle which had a smooth entarance.
