Abstract: A palm handheld computer has a host, an enclosure and a water proof structure. The host has at least two connection slots allowing various types of function cards to be plugged. The host is housed within the enclosure. The enclosure includes a front panel and a rear panel, and the water proof structure is disposed between the front and rear panels to provide water proof. Thereby, function cards with various specifications can be applied to the host to expand access capacity of the handheld computer. Further, the water proof mechanism allows the handheld computer to be used outdoors or in a humid environment.