CTR(Click-Through Rate) prediction can be used to improve users' satisfaction with respect to the presented online advertisements(ads) and support effective advertising.CTR prediction is the basis for personalized recommendation of online ads.It is also necessary to recommend ads and predict their CTRs for the users that have no historical click-through records. In this paper,we adopted BN(Bayesian network),an important probabilistic graphical model, as the framework for representing and inferring the similarity and the corresponding uncertainty of the behaviors in ad search of different users.First,we constructed the BN to reflect the similarity between users by means of statistic computations on the historical records of user's ad search.Then,we measured the behavior similarity between the users with click-through records and those without records quantitatively based on the mechanism of BN' s probabilistic infer- ences.Consequently,we predicted the CTRs of ads with respect to the users without historical click-through records,in order to provide a metric for ad recommendation.We made experiments on the training data of Tencent CA from KDD Cup 2012-Track 2 and tested the effectiveness of our methods.