Twitter has shown its greatest power of influence for its fast information diffusion. Previous research has shown that most of the tweets posted are truthful, but as some people post the rumors and spams on Twitter in emergence situation, the direction of public opinion can be misled and even the riots are caused. In this paper, we focus on the methods for the information credibility in emergency situation. More precisely, we build a novel Twitter monitor model to monitoring Twitter online. Within the novel monitor model, an unsupervised learning algorithm is proposed to detect the emergency situation. A collection of training dataset which includes the tweets of typical events is gathered through the Twitter monitor. Then we manually dispatch the dataset to experts who label each tweet into two classes: credibility or incredibility. With the classified tweets, a number of features related to the user social behavior, the tweet content, the tweet topic and the tweet diffusion are extracted. A supervised method using learning Bayesian Network is used to predict the tweets credibility in emergency situation. Experiments with the tweets of UK Riots related topics show that our procedure achieves good performance to classify the tweets compared with other state-of-art algorithms.