To study the influence of filamentous fungi on the sludge dewaterability is very significant for the development of biological treatment methods for enhancing sludge dewaterability. In this study, filamentous fungi capable of enhancing sludge dewaterability were isolated from sewage sludge and the related mechanisms responsible for the sludge dewaterability enhancement were investigated. A filamentous fungus Mucor circinelloides ZG-3 was successfully isolated from sludge, and sludge dewaterability could be drastically improved by this fungus. Further study revealed that the enhancement of sludge dewaterability was influenced by inoculation method, inoculum size and solid content of sludge. The optimal inoculation method was mycelia inoculation, the optimal inoculum size was 10%, and the optimal solid content of sludge was about 4%. Under the optimized conditions, the specific resistance to filtration (SRF) of sludge could be decreased by 75.1% after being treated by M. circinelloides ZG-3. After the treatment, the COD value of sludge supernatant was only 310 mg x L(-1), and the treated sludge still exhibited good settleability. During the treatment of sewage sludge by M. circinelloides ZG-3, the mechanisms responsible for the sludge dewaterability enhancement included the degradation of sludge extracellular polymeric substances (EPS) and the decrease of sludge pH. Therefore, the treatment of sewage sludge using M. circinelloides ZG-3 is a useful and novel method for sludge conditioning.