Using micorfauna to reduce excess sludge is a potentially effective ecological technology and scaling the rate of sludge reduction rate is the first step. A method to scale the rate of sludge reduction caused by microfauna was proposed, and comparison of sludge reduction rates induced by four microfaunas was carried out. The principle of this method is based on the change of carbon forms. The rate of sludge reduction was correlated with the rate at which solids were changed into liquid and gas. Four microfaunas, including Aeolosoma hemprichi, Daphnia magna, Tubifex tubifex and Physa acuta, were cultured with sterilized sludge in a covered sterilized bottle and were then isolated from the atmosphere above the liquid phase. The rates of sludge reduction using the four microfaunas were 0.8, 0.18, 0.54 and 0.1 mg-sludge/(mg-Microfauna d), respectively, changing with the microfaunas' phylum or class and body size. Based on the change of carbon (C) forms, the proposed method produced accurate results similar to those produced using the direct measuring method.