A home-made double-clad Tm3+ -doped silica fiber laser with over 150-W output power is reported. Pumped with a diode bar at 793nm, the fiber laser is operated at 2.04μm with a slope efficiency of 56.3%. A quantum efficiency of 〉 140%is achieved owing to the efficient cross-relaxation process in heavily Tm3+-doped fibers. By using different fiber-cooling techniques, the slope efficiency shows a deviation over 10%. The influence of output coupling on the laser characteristics is also investigated.