With the rapid development of workflow technology, many process definition languages appeared, forming many workflow process definition documents which were described by a variety of languages. How to implement conversion among these process definition documents is becoming an urgent problem. This paper mainly researches the rules during the conversion process, achieves the definition of transformation rules and implements the definition with the Java Rule Engine-Drools. In order to validate this rule definition, the transformation rule data bases are created from XPDL to BPEL and from BPEL to XPDL, and the transformation of the instance is achieved. The results show that the method of transformation rules proposed in this paper is valid.