In high‐power AlGaAs window structure lasers with a window grown on facets (WGF) structure, dependence of window effects on Al content of the window layer is examined. When the Al content of the window layer is more than that of the cladding layer, the window effects are found. But, in other cases, the window effects do not occur. A calculation of a carrier leakage from the active layer to the window layer is performed. From the result, we deduce that the reduction of the window effects is caused by the carrier leakage. In the WGF laser with the confirmed window effects, a maximum output power of 350 mW is achieved and highly reliable operation under 100 mW at 50 °C beyond 10000 h is attained.