The tree quorum algorithm (TQA) uses a tree structure to generate intersecting (tree) quorums for distributed mutual exclusion. This paper analyzes the number of messages required to acquire a quorum in TQA. Let i be the depth of the complete binary tree used in TQA, and let M
i be the number of messages required to acquire a quorum or to determine that no quorum is accessible. We discuss M
i as a function of i and p, where p (½<p<1) is the probability that each site is operational. Let C
i denote the average number of sites in the quorum that TQA finds. The analysis shows that, although both M
i and C
i increase without bound as i increases, M
i/C
i approaches to 1+p/p as i increases. According to the result, an approximate close form for M
i is derived
