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Abstract─ This paper evaluates the performance of X-MAC protocol when it is equipped with CA (Collision Avoidance) algorithm with an aim to maximally randomize transmissions in overcrowded networks. X-MAC protocol, one of typical asynchronous MAC protocols allowing each node to independently and periodically sleep to save energy, should send a sequence of short preambles until an early acknowledgement comes back. When the sleep period is long enough, the performance of X-MAC protocol would rapidly deteriorate due to lengthy collision duration. This paper extends a conventional mathematical model for X-MAC protocol to include the effect of CA algorithm. The extended model predicts that CA algorithm can improve the performance of X-MAC protocol by upto 25% in 40-node networks.