Based on the concept of Cloud Computing, Cloud Manufacturing which refers to a new networked manufacturing mode is proposed by LI Bo-hu. In this new mode, virtual resources are organized to meet the requirements of various manufacturing services for users over network and Cloud Manufacturing platform. This paper focuses on the virtualization of resources and analyses the factors which affect the classification of virtual resources in Cloud Manufacturing first, and a conceptual classification is given. Then some factors affect the result of task assignment are introduced, and a simple example shows the effect of these factors to task assignment. Based on these factors which include quantitative, human activity, logistics and granularity, a modeling process and a rough model of virtual resource are built.