Abstract: The flavour of hydrolysate from Oreochromis niloticus was improved with mixed lactic acid bacteria fermentation of Streptococus thermophilus and Lactococcus bulgaricus. Response surface methodology was used to study the effects of fermentation time (X1), fermentation temperature (X2) and inoculation volume (X3) on the sensory scores of hydrolysate flavour. The volatile components of hydrolysate were evaluated by simultaneous distillation-extraction (SDE) and gas chromatography-mass spectrometry (GC-MS). The results indicated that the optimal conditions of fermentation were X1=9 h, X2=45 ℃ and X3=16%. The predicted value ofresponse optimal conditions was Y=4.1. After fermentation, the number of volatile components of hydrolysate increased from 27 to 33, which were mainly ester. Besides, other flavour compounds such as caprylic acid and palmitic aldehyde were also produced. The lactic acid bacteria fermentation could remove fish odour of protein hydrolyzates but its debittering effect was not obvious.