Activated sludge process to treat municipal and industrial wastewater produces huge amounts of excess sludge. Chemical condition has been employed widely to improve sludge mechanical dewatering, but the cost is high, thus it is very important to find cheap and effective conditioners. This paper studied the improvement of sludge dewaterability with coal fly ash modified by sulfuric acid (MCFA). Through orthogonal experiments with specific resistance to filtration (SRF) as the target index, acid concentration and soaking time were verified to be the important influencing parameters in coal fly ash modification. The optimal modification conditions were: acid concentration, 4 mol l(-1); ratio of acid to coal fly ash, 5:1 ml g(-1); soaking time, 3 h. After modification the specific surface area of coal fly ash increased from 2.810 to 3.376 m(2) g(-1). The dewaterability and the settleability of the conditioned sludge were investigated with vacuum filtrating dewatering tests, centrifugal dewatering tests and settling experiments. The results showed that SRF of the sludge significantly decreased with coal fly ash addition, and the MCFA showed much stronger conditioning capacity than the raw coal fly ash (RCFA). Under a MCFA dosage of 273%, the SRF of the sludge decreased from 1.86 x 10(13) to 4.23 x 10(11) m kg(-1), and the filter cake moisture decreased from 86.90% to 56.52%. The sludge conditioning mechanisms with MCFA mainly included improving floc formation through charge neutralization and adsorption bridging and providing the water transmitting passages by skeleton builder. (C) 2010 Elsevier B.V. All rights reserved.