Recent studies have been conducted to indicate the ineffective usage of licensed bands due to the static spectrum allocation. In order to improve the spectrum utilization, the cognitive radio (CR) is therefore suggested to dynamically exploit the opportunistic primary frequency spectrums. How to provide efficient spectrum handoff has been considered a crucial issue in the CR networks. Existing spectrum handoff algorithms assume that all the channels within the network can be correctly sensed by the CR users in order to perform appropriate spectrum handoff process. However, this assumption is considered impracticable in realistic circumstances primarily due to the excessive time required for the CR user to sense the entire spectrum space. In this paper, the partially observable Markov decision process (POMDP) is exploited to estimate the network information by partially sensing the frequency spectrums. A POMDP-based spectrum handoff (POSH) scheme is proposed to determine the optimal target channel for spectrum handoff according to the artially observable channel state information. By adopting the policy resulted from the POSH algorithm for target channel selection, minimal waiting time at each occurrence of spectrum handoff can be achieved. Numerical results illustrate that the proposed POSH scheme can effectively minimize the required waiting time for spectrum handoff in the CR networks.