The diagnosis of faulty processors plays an important role in multiprocessor systems for reliable computing, and the diagnosability of manywell-known networks has been explored. Zheng et al. showed that the diagnosability of the n-dimensional star graph Sn is n − 1. Lai et al. introduced a restricted diagnosability of multiprocessor systems called conditional diagnosability. They consider the situation when no faulty set can contain all the neighbors of any vertex in the system. In this paper, we study the conditional diagnosability of Cayley graphs generated by transposition trees (which include the star graphs) under the comparison model, and show that it is 3n − 8 for n ≥ 4, except for the n-dimensional star graph, for which it is 3n − 7.