In this paper, a new acoustic confidence measure of automatic speech recognition hypothesis is proposed and it is compared to approaches proposed in the literature. This approach takes into account prior information on the acoustic model performance specific to each phoneme. The new method is tested on two types of recognition errors: the out-of-vocabulary words and the errors due to additive noise. An efficient way to interpret the raw confidence measure as a correctness prior probability is also proposed in the paper. (C) 2005 Acoustical Society of America.