The present invention provides a housing assembly of a portable electronic device. The housing assembly includes a first housing, a second housing and a third housing with two holes and two openings thereon. When the first and second housings are assembled together, the third housing is in an inner space defined by the first and second housings. A U-shape slider is slidably disposed in the third housing through the two openings. When the third housing is in the inner space, two arms of the U-shape slider have slots to accommodate protrusions of the first and second housings respectively.