Message Authentication Codes are very important tools to protect data integrity. XOR-MAC is defined by Bellare, which also give a security analysis for it. But security analysis for XOR-MAC they gave is complex. In this paper, we use Game-Playing technique to give a new security analysis for XOR-MAC. We prove XOR-MAC secure, quantifying an adversary’s success probability of distinguishing XOR-MAC from purely random function in terms of the quality of the block cipher as a pseudorandom permutation.