Abstract: A portable electronic device includes a base module with a housing defining upper and lower sliding grooves at each of two opposite side walls of the housing, and a system module mounted slidably on the base module through a pair of sliding units. Each of the sliding units includes an upper rolling member that is received in the upper sliding groove and that is in rolling contact with a respective one of the side walls, and a lower rolling member that is received in the lower sliding groove and that is in rolling contact with a respective one of the side walls.