A fastening unit includes a hole-defining unit and a fastening component. The hole-defining unit is adapted to be provided in a surface of a first body, and includes a non-circular fastener hole and at least one first engaging part. The fastening component is adapted to be fixed to a second body, is extendible in an axial direction into the fastener hole, and includes a post body and at least one surrounding member. The post body has a non-circular cross-section along a plane transverse to the axial direction, and is provided with at least one second engaging part equal in number to and having a structure complementing the first engaging part. When the fastening component is pushed into the fastener hole to interconnect the first and second bodies, the first and second engaging parts cooperate so that the fastening component is prevented from rotating in the fastener hole.