As information and communication technology develop rapidly, there is an increasingly perceived vision that cloud computing will become a utility just like gas, water and electricity. Cloud computing, as a daily service, will be provided through the network and be consumed elastically. To deliver it efficiently on user's demand, automatic provisioning is essential. The IaaS provisioning has to deal with the allocation, configuration and activation of heterogeneous resources, which involve different virtualization technologies, as well as physical hosts, storage, software and network equipment in the cloud. Moreover, a cloud consisting of datacenters in different locations complicates the service provisioning strategies and processes. Facing these challenges, in this paper an intelligent provisioning engine with the features of feasibility, flexibility, re-usability and configurability is proposed to realize the automatic provisioning of on-demand cloud computing services.