Due to the diversity of resources and different management requirements, cloud management is faced with great challenges in complexity and difficulty. For constructing a management system to satisfy a specific management requirement, redeveloping a solution based on existing management system is usually more practicable than developing the system from scratch. However, the difficulty and workload of redevelopment are also very high. In this paper, a runtime model based approach is presented to managing diverse cloud resources. First, the runtime model is constructed for each type of cloud resources based on their management interfaces. Second, the composite runtime model is build for all managed resources through merging their runtime models. Third, cloud management is setup to meet specific requirements through model transformation from the composite model to the customized models. Additionally, based on OpenStack and Hyperic, a runtime model based management system is implemented to manage the hardware and software resources of virtual machines with the proposed approach. The results prove that new approach is feasible and effective.