A noise estimator was presented in this paper by modeling the log-power sequence with hidden Markov model(HMM).The smoothing factor of this estimator was motivated by the speech presence probability at each frequency band.This HMM had a speech state and a nonspeech state,and each state consisted of a unique Gaussian function.The mean of the nonspeech state was the estimation of the noise logarithmic power.To make this estimator run in an on-line manner,an HMM parameter updated method was used based on a first-order recursive process.The noise signal was tracked together with the HMM to be sequentially updated.For the sake of reliability,some constraints were introduced to the HMM.The proposed algorithm was compared with the conventional ones such as minimum statistics(MS)and improved minima controlled recursive averaging(IMCRA).The experimental results confirms its promising performance. A noise estimator was presented in this paper by modeling the log-power sequence with hidden Markov model (HMM). The smoothing factor of this estimator was motivated by the speech presence probability at each frequency band. This HMM had a speech state and a nonspeech state, and each state consisted of a unique Gaussian function. The mean of the nonspeech state was the estimation of the noise logarithmic power. To make this estimator run in an on-line manner, an HMM parameter updated method was used based on a first -order recursive process.The noise signal was tracked together with the HMM to be updated updated. For the sake of reliability, some constraints were introduced to the HMM. The proposed algorithm was compared with the conventional ones such as minimum statistics (MS) and improved minima controlled recursive averaging (IMCRA). The experimental results confirms its promising performance.