Abstract: The methods for testing pore size of various porous materials have been studied by liquid-liquid system. By using ortho-butanol-water system with low interracial tension and ortho-butanol as impregnating medium while water as infiltrating medium, only 3.6×10^-4~3.6×10^-1 MPa testing pressure is necessary for measuring the pore size of 10~0.01μm with measuring accuracy of 10~1μm 2% and 1~0.01μm, 5%. Liquid-liquid method has the following advantages that very fine pore size of 0.01μm grade can be measured by low testing pressure of 3.6×10^-1MPa, and the structure of testing apparatus can be greatly simplified.