Grid computing is to enable coordinated resource sharing in dynamic, distinct virtual organizations, which are consisted of different grid entities from traditional organizations. Grid security must ensure secure and authorized access to grid services in such dynamic, heterogeneous and unpredictable gird environment. The main current security solution is applying trust as an important factor to the gird computing to realize cooperation between unknown entities.However, most of the research work inherites the conception of trust from Peer-to-Peer network and is not accurately satsified the requirements in grid environemnt. In this paper, we propose a multi-virtual organizations trust evaluation method. The unique feature of our trust model is that introducing VO trust relationship into traditional grid entities, with which we can more effectively and dynamically share grid resources. In addition,the new method uses clustering analysis to evaluate grid entities’ trust in VOs and a weighted path to compute transitivity of trust.The simulation result shows that it improves the overall??performance of grid system.