Abstract: The issue of fault diagnosis for mobile hosts in ad-hoc networks was studied. The cluster-based comparison diagnosis algorithm is proposed to use in hierarchical ad-hoc networks so as to replace comparison-based fault diagnosis algorithm. Making use of the concentrative control function of cluster heads within clusters, this algorithm can reduce greatly the overhead expenses for the diagnosis to a system in comparison to the comparison-based fault diagnosis algorithm and break through the limitation that the network topology must be fixed in the process of diagnosis. Simulation results indicate that it has a better performance with lower cost than the comparison-based fault diagnosis algorithm.