In this research, we studied the effects of poly ether sulfone (PES) dope concentration varied from 22 to 28 wt. % on the morphologies and mean surface pore sizes of the PES hollow fiber substrates. In addition, the relations between the mean surfaces pore sizes of the PES substrate to the gas separation performances of the poly ether sulfone/poly dimethyl siloxane (PES/PDMS) hollow fiber composite membranes were explored. Results showed that as the PES concentration in the dope solution increased, both the mean pore size of the PES substrate and the gas permeance of PES/PDMS composite membrane decreased. SEM pictures showed that the dense layer of the PES substrate became apparently thicker and denser with the increase in PES dope concentration.