Today, concurrence with information technology (IT) progress, a novel issue has been introduced which is named as" cloud computing". The greatest obstacle that often is considered for acceptance of cloud services is lack of customers' trust to service providers. In this research, the problem of creating trust in cloud services has been evaluated and a model is presented to perform a comprehensive prioritization about services which are provided by cloud providers and let users to compare and choose proper service providers (in proportion with their needs) and in the other hand, enable service providers to increase their services' quality through an intact competition. Based on the subjective logic trust model, this paper combines direct experiences, reputation values, satisfied interaction feedback and risk factor. The simulations performed on this model confirm its correctness and its rationality of our trust model.