The present invention provides a test key array including a lower conductive pattern, and the lower conductive pattern includes a plurality of first L-shaped traces parallel to each other, an upper conductive pattern, where the upper conductive pattern includes a plurality of second L-shaped traces parallel to each other, the lower conductive pattern crosses to the upper conductive pattern, and a plurality of cross regions are defined between the lower conductive pattern and the upper conductive pattern, and a plurality of conductive plugs, disposed on parts of the cross regions, electrically connecting to the lower conductive pattern and the upper conductive pattern.