ABSTRACT: Cloud Computing is one the famous technology that are used today. It provides on-demand access through internet to computing different services. Privacy and Security are big issues in Cloud Computing. The cloud environment is a large open distributed system. It is important to preserve the data, as well as, privacy of users. Access Control methods in Cloud Computing ensure that only authorized the user to access system and data that helps to increase the security in the Cloud. Nowadays researchers devoted their work access control method to enhance the security on Cloud. Various access control models are in use, including the most common Mandatory Access Control (MAC), Discretionary Access Control (DAC) and Role Based. Access Control (RBAC).This paper proposes the security enhancement in Role Based Access Model using Reference Ontology to restricts the number of user per role, number of transaction per user and adds the feature of backup and restoration. The advantages to add these features to enhance the more security on Cloud Computing and data will never loss in case of Cloud crash.