An oil pan for an internal combustion engine having an engine lubrication system. The oil pan comprises a shallow bottom section and a sump. The shallow bottom section includes an inclined surface which gradually becomes lower in level toward the sump and toward the downstream of the oil flow caused by the rotation of the crankshaft. Therefore, the lubrication oil dropped on the shallow bottom section is effectively returned into the sump through the lower side of the inclined surface under the action of a force caused by the rotation of the crankshaft or the like.