A cylinder block for an engine is formed of a cylinder block body and a lower block which are cast from aluminum alloy and are fastened together to support therebetween a crankshaft of the engine. The upper half of a bearing portion for supporting the crankshaft is provided in the cylinder block body and the lower half of the bearing portion is formed by a bearing cap cast in the lower block. A method of manufacturing the cylinder block includes the steps of casting the cylinder block body and the lower block with the bearing cap cast in the lower block, fastening together the cylinder block body and the lower block by bolts, performing primary stage finishing of the bearing portion with the cylinder block body and the lower block kept fastened together, releasing the lower block from the cylinder block body after the primary stage finishing of the bearing portion and permitting the lower block to deform under stress produced by shrinkage of aluminum alloy, fastening again the cylinder block body and the lower block together by the bolts, and performing final stage finishing of the bearing portion with the cylinder block body and the lower block kept fastened together.