An adjustment mechanism for altering a height of a baby playpen from the ground to a bottom of the baby playpen is disclosed. The adjustment mechanism includes a foldable skeleton, a seat fabric slideably put on the foldable skeleton to construct a plurality of side surface and a bottom surface for forming an accommodation space, and a height adjustment positioning mechanism disposed between the foldable skeleton and the seat fabric thereby altering the height of the bottom surface from the ground by adjusting the seat fabric.