This paper gives an account on the significance of and the present situation and problems in the determination of absorption isotherm and decolorizing power of me-thylene blue with powdered activated carbon. Results of the determination of absorption isoterm of methylene blue with different powdered activated carbon made by zine chloride process showed that the determined absorption isotherms have the same characteristics as implied in Freundliek’s equation. They are approximately parallel lines when expressed in the natural logarithm, which provides theoretical bases for a better method in determining absorption isotherms than the present single point method. Tests proved that with powdered activated carbon made by zinc chloride process, decolorizing power of methylene blue can be approximately determined in the very first performance. This furnishes a simple and convenient method for determining the decolorizing power of methylene blue with powdered activated carbon.