A board to board connector includes a connector housing and a number of contacts mounted to the connector housing. The connector housing includes a base and a pair of side walls extending from the base along a vertical direction. Each side wall includes a number of first mounting walls and a plurality of second mounting walls alternatively arranged along a longitudinal direction perpendicular to the vertical direction. The contacts are fixed to the first mounting walls and each contact is positioned by the adjacent two second mounting walls along the longitudinal direction. A thickness of each second mounting wall is variable along the vertical direction for avoiding irrecoverable deformation or damage thereof.