Abstract: The present invention relates to a foldable playpen which comprises a main frame constructed of top articulated bars and upright bars, which is provided with a covering to form an enclosure so that a baby can be left to play therewithin. The primary feature of the present invention resides in that the main frame includes a bottom sub-frame structure pivotably mounted at the bottom of the main frame. The bottom sub-frame structure includes at least first, second, third and fourth bottom side-bars, and a folding mechanism. The first, second, third and fourth bottom side-bars are arranged in a pivotingly foldable relationship with the folding mechanism, whereby the present invention can be conveniently folded into a compact package when it is not in use or extended to an erect enclosure for allowing a baby to play therewithin.