Abstract: In mobile distributed databases, the communications between sites only provide weak connectivity. To improve the efficiency of mobile computers, optimistic replication approaches are used as common methods for transaction processing. The shortcomings of the approaches are that they rely on user’s knowledge about the applications data and sites behavior. It is difficult for the approaches to be adaptive to the dynamic changes of network connection and configuration in mobile environments. In this paper, a probability based method for transaction processing in mobile distributed databases is given for the first time. It adopts the quality of service specification to achieve transactional consistency dynamically and accomplishes transaction processing by the collaboration between applications and systems. The experimental results show that the probability based method outperforms the simple optimistic replication method.