A rotating power tool has a transmission assembly having a transmission casing, a common annulus, at least two planetary gear assemblies and a thrust washer assembly. The common annulus is tubular, is mounted securely in the transmission casing and has an inner surface and multiple teeth. The planetary gear assemblies are connected in series in the common annulus, and each planetary gear assembly has a sun gear, a planet carrier and at least three planetary gear assemblies. The planet carrier is an annular bracket. The planetary gear assemblies are mounted around and engage a sun gear and the teeth in the common annulus. The thrust washer assembly is mounted between adjacent planetary gear assemblies. Consequently rotation of the transmission assembly is smooth and stable. The thrust washer assembly damps reactive force to prevent damage and guarantee normal operation of the hand-held rotating power tool.