A computer enclosure includes a shell body, and an expansion card retention assembly. The shell body includes a main chassis, and a side wall perpendicular to the main chassis. The side wall defines an elongated through hole extending along a first direction perpendicular to the main chassis. The expansion card retention assembly includes a first and second expansion frames fixed on the side wall. The first expansion frame includes a main board with a length in the first direction less than that of the through hole, and is configured for fixing a first expansion card. The second expansion frame includes an elongated main plate extending along the first direction, and a first support platform perpendicularly extending from an end of the main plate. The main plate and the first support platform are configured for matingly engaging with an expansion card bracket of a second expansion card.