A polishing head of a chemical mechanical polishing apparatus includes a housing moving up and down, a base assembly connected to a bottom of the housing to support the housing, a membrane on a bottom of the base assembly and a retainer ring surrounding the membrane and connected to the bottom of the base assembly, the membrane including a pressing portion to adsorb and press a substrate, a first partition on the pressing portion and extending from an edge of the pressing portion along a height direction, a first horizontal extending portion extending from an upper end portion of the first partition toward a center of the membrane, and a second horizontal extending portion from the upper end portion of the first partition toward the center of the membrane, the second horizontal extending portion being above the first horizontal extending portion and including a curved portion expanding by pneumatic pressure.