This paper starting with the present status of enterprise interoperability, focuses on enterprise interoperability of SMEs (Small and Medium-sized Enterprises), and based on a SMEs oriented framework of enterprise interoperability proposes an approach of process level interoperability. The approach appropriate for SMEs mainly solves two types of interoperability involving processes’ interior in the process level. Life cycle of process level interoperability is redesigned and reorganized. In each phase of the life cycle a comprehensive solution is given. In final, this paper gives a case study to describe and enhance the approach.