Continuous-wave oscillation characteristics of a 1. 06- mu m Nd**3** plus :YAG laser with a long optical-fiber resonator were investigated. Output power of 27 mW was obtained for a low-loss graded-index multimode fiber 83 m in length. Laser oscillations with a cavity length up to 770 m were attained by using a graded-index fiber. The mode pattern of the output beam from a mirror coated at the fiber end face exhibited a typical speckle pattern. A single-mode fiber with a length of 1 m was also used as the laser cavity, and a clear Gaussian beam with an output power of 1 mW was observed.