Abstract: A target channel visiting scheme which visits target channels in a descending order of channel mean vacant time duration is proposed for proactive decision spectrum handoff in cognitive radio. It proofs that this scheme result in minimum probability of handoff failure when the channel vacant time duration follows uniform distribution, Rayleigh distribution or Weibull distribution. Simulations show that the probability of handoff failure of this scheme is far lower than that of the random target channel visiting scheme. Moreover, when tens of samples of channel vacant time duration can be obtained, even though there is estimation error, the probability of handoff failure of the proposed scheme is very close to the optimal minimum probability.