Business and organization environments need using numerous processing, saving and software resources for achieving their determined goals. Genesis of Cloud Computing, transform method of access to the information processing and saving resource. Having easy accessing features through network communication standard mechanism and automatic increasing resource mechanism, cause great interest in organizations to use Cloud services. However, choosing suitable service despite increasing variety of services in terms of price, place and quality of service parameters, make the cloud services selection process so complex. One of the requirements for choosing the right cloud service is identifying the factors influencing the selection and estimation of the importance of them. This research has tried to follow a mixed approach and use systematic literature review and fuzzy Delphi method in the form of an exploratory plan, identifies the effective factors in choosing the organizations’ cloud service and rank these factors with the help of the best-worst method. In the first step, by systematically reviewing the literature in this field and also comparatively reviewing the three industrial standards of cloud service level agreements, 24 elements affecting cloud service selection were identified. In order to expand the existing body of knowledge, validate and refine the factors affecting the choice of cloud service, Fuzzy Delphi method was used. Based on the consensus of experts, 20 factors affecting the choice of organizations’ cloud service were approved and finalized in the form of 4 main components. At the end, the factors confirmed from the previous stage were weighted and ranked by the best-worst method. The results showed that performance, security, environmental and organizational components are the most important in choosing a cloud service. Of the 20 factors influencing cloud service selection, availability factors weighing 0.137, reliability weighing 0.134, response time/delay weighing 0.122, governance weighing 0.096, and capacity weighing 0.072 have the highest weight respectively.