Abstract: In order to screen a high acid- producing and acid- resistant strain,Lactobacillus casei LC2 W was subjected by Ultraviolet( UV),Nitrosoguanidine( NTG),Diethyl sulfate( DES) respectively,and then treated with simulated gastric fluid.The results showed that the mutation strain LC2W- NTG-12 and LC2W- UV-11 which were subjected by 0.3 g / L NTG for 30 min and UV treatment for 30 s exhibited high acid- producing and acid- resistant.In the same amount of inoculation,the acidity of the fermented by the parent strain and mutation strain were 76.6 °T and 112.7 °T,98.2° T after 24 h,respectively. The acid- production rate of mutation strain was improved 47.13%,28.20%.In addition,when the strains were treated with the acid( p H = 1.5),the mortality rate of parent stain and mutation strain were 76.44% and 52.06%,56.36% after 2 h,respectively. The acid- resistant rate of mutation strain was improved 24.38%,20.08%.But the effect of mutation strain LC2W- DES-33 which was subjected by 0.16% DES for 30 min was not obvious.The acid- production rate of mutation strain was improved only 14.10% compared with the parent strain.This implied that NTG mutation can effectively improve the acid- producing and acid- resistant of the Lactobacillus casei LC2 W.