Lack of trust has often been considered as a crucial barrier to the adoption of electronic commerce. To address this problem, a trust model is proposed in this paper. Since uncertainty is the nature of trust, it is necessary for us to take uncertainty into account during modeling trust. Distinguished from other trust models, a novel approach is employed to describe uncertainty in the proposed trust model, namely the cloud-model. Since the cloud-model is quite new and still in the progress of development, it can not be applied into modeling trust directly, hence we have to extend it firstly. Then we present an extended-cloudmodel- based trust model (abbreviated as ECM-based trust model or ECMBTM). To prove the advantage of the proposed trust model, simulation experiments are carried out, and the ECMBTM is compared quantificationally with three typical trust models. Furthermore, we develop a trust evaluation system based on the ECMBTM and are applying it in a real B2B system.