Two kinds of pyrolysis carbon membranes for gas separation were prepared by control of preparation conditions. One was characterized by higher selectivity and another by higher permeability. The preparation process of the carbon membranes consists of pyrolysis and activation steps. The first step was conducted under inert gas and the second step in an oxidation environment. A thermosetting phenol formaldehyde resin membrane was chosen as the precursor. Preparation conditions were important factors for the structures and properties of the resulting carbon membranes. Pore size distribution was measured and morphology was observed. Effects of membrane structure on characteristics were discussed.