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Ab initio calculations are carried out to study the anchimeric assistance of the nitroso group on the βcarbon electrophilic center of nitramines and nitrosoureas.The molecular structures are alloptimized at the RHF/6-311G(d,p) theoretical level,and intrinsic reaction coordinate (IRC) and molecular orbital analysis are performed to investigate the reaction process.Because the C=O group delocalizes the π electrons in the molecular system,the negativecharges on the nitroso oxygen atom of nitrosoureas are much lower than that of nitrosamines.Consequently,nitrosamines are more favorable to undergo the anchimerie assistant process.The activation energy and the relative energy,of the intermediates are calculated,and the results show that the alkylating activity of nitrosamines is obviously higher than that of nitrosoureas.This result indicatesthat the difference of the carcinogenicity between nitrosamines and nitrosoureas can be attributed to the distinguishing anchimeric assistant activity of the nitroso group on the β-position carbon.