Ozonation of nitrophenols and subsequent biodegradability were studied. Nitrophenols were easily decomposed by ozonation, but total organic carbon (TOC) could be reduced only to about 30-50% during an ozonation period of 120 minutes. The ozonated solution, after being mixed with activated sludge culture, was incubated for 4 days at 25°C, and biodegradability was then assessed by dissolved organic carbon (DOC) reduction. DOC of a non-ozonated solution was only slightly reduced even after 4 days incubation, indicating poor biodegradability. For the ozonated solution of nitrophenols, DOC removal more than 80% was achieved with a resulting DOC of less than 10 mg·l-1. Ozonation thus greatly improves the biodegradability of refractory nitrophenols.