To provide an oil pump mounting structure for an engine. The engine includes a crankcase rotatably supporting a crankshaft, an oil pan connected to a lower portion of the crankcase, and an oil pump interlocked for rotation with the crankshaft. The oil pump mounting structure lowers the center of gravity of the engine and improves the suction efficiency and maintenance characteristic of the oil pump. A pump housing of the oil pump is removably mounted on a mounting portion provided on the bottom of an oil pan.