A protocol for the copper-catalyzed N-methylation of sulfoximines with di-tert-butyl peroxide (DTBP) was developed. This protocol has good functional group tolerance leading to N-methylated sulfoximines in moderate to good yields. Besides, N-ethylation of sulfoximines was achieved in the presence of bis(1,1-dimethylpropyl)peroxide as the ethylating agent under a standard procedure.