An overflow pipe assembly of a bathtub includes an overflow pipe having a horizontal fitting end which has outer threads on outside thereof. A flange extends from the outside of the fitting end. A buffering ring is mounted to the fitting end and clamped between the flange and the wall of the bathtub. Two slots are defined axially in the passage of the fitting end. A locking ring is threadedly connected to the outer threads of the overflow pipe. A cap is mounted to the fitting end and has a peripheral wall which includes multiple drain holes. The cap has a tubular portion inserted into the passage. The tubular portion has a central hole which communicates with apertures defined through the wall of the tubular portion. The tubular portion has two protrusion which are engaged with the slots of the fitting end.