Porous gamma-alumina with a bimodal pore size distribution has been developed by adding nanosized polystyrene beads to boehmite sol as templating units. The primary pore diameter is in the range of 4-6 nm and the secondary pore diameter is ca. 50 nm with minor pore shrinkage. The unsupported gamma-alumina with different porous structures are characterized using thermogravimetric analysis, Fourier transform infrared spectra, X-ray diffraction, N-2 adsorption/desorption, and transmission electron microscopy. gamma-alumina with a bimodal porous structure shows reduced transport resistance compared with gamma-alumina with a unimodal porous structure in the dye adsorption test. Although the thickness of gamma-alumina thin layer increases when more secondary pores are generated, a gamma-alumina membrane with a bimodal pore size distribution shows diminution of transport resistance in the water permeability study also.