To elucidate the effects of CaO on nitrogen transformation during sludge pyrolysis, transformation behavior from char N into NOx precursors during pyrolysis of soybean protein (SP) with CaO at 600-700 degrees C was investigated. Results showed that CaO inhibited the transformation of pyridine N and quaternary N into HCN and promoted HCN conversion into NH3 at 600-700 degrees C. CaO inhibited the conversion of protein N and tar N into NH3 at 600 degrees C but promoted it at 700 degrees C. NOx precursor yield was the lowest when SP was pyrolyzed with CaO/N of 5.5 at 600 degrees C (reduced by 11.66% compared with raw SP pyrolysis).