A method of phoneme boundary detection based on phoneme posterior probability and Hierarchical Agglomerative Clustering(HAC) is presented. According to this method, phoneme posterior probabilities should first of all be got by Temporal Pattern(TRAP), and then HAC algorithm is chosen to cluster the phoneme posterior probability. Finally, a reasonable threshold can be obtained by all loss function values, and the number of clusters and the phoneme boundaries can be determined by the threshold. The experimental results show that this method is efficient and bears a good detection performance; and the phoneme posterior probabilities are more suitable for phoneme boundary detection than the Mel-Scale Frequency Cepstral Coefficients(MFCC).