Co-composting of sewage sludge with coal fly ash was carried out for evaluating the effect of coal fly ash on nutrient transformations during sludge composting. Dewatered anaerobically-digested sewage sludge was mixed with sawdust used as a bulking agent at 2:1 (w/w), and the mixtures were amended with coal fly ash at 0, 10, 25 and 35% (why) and composted for 100 days. Addition of coal fly ash raised the pH of the sludge compost throughout the composting period: but significant inhibition of decomposition activity occurred only at 35% ash amendment level. Soluble organic carbon and total C decreased according to composting time, whereas total N showed an opposite trend for all treatments. Addition of coal fly ash did not affect the carbon cycle significantly except at an ash amendment level of 35%. Coal fly ash amendment caused a significant loss of NH4-N only during the thermophilic phase. Addition of coal fly ash also inhibited the nitrification process and phosphorus transformation as indicated by the lower soluble NO3-N and PO4-P contents in ash-amended sludge composts. The C/N ratios in solid and aqueous phases decreased with composting time but did not show any significant difference after 63 days of composting for all treatments indicating that ash amendment did not affect compost maturity.