In cloud computing, security is an important issue due to the increasing scale of users. Current approaches to access control on clouds do not scale well to multi-tenancy requirements because they are mostly based on individual user IDs at different granularity levels. However, the number of users can be enormous and causes significant overhead in managing security. RBAC (Role-Based Access Control) is attractive because the number of roles is significantly less, and users can be classified according to their roles. This paper proposes a RBAC model using a role ontology for Multi-Tenancy Architecture (MTA) in clouds. The ontology is used to build up the role hierarchy for a specific domain. Ontology transformation operations algorithms are provided to compare the similarity of different ontology. The proposed framework can ease the design of security system in cloud and reduce the complexity of system design and implementation.