Software as a service (SaaS) is an emerging software framework in which business data and logic typically integrate with other applications. It requires a unified subscriber to describe SaaS to make for easy integration; however, SaaS provides services to different tenants by running only one instance. In order to satisfy personalized needs from different tenants, the business logic becomes correspondingly complex. As this logic is cumbersome to reveal to every individual tenant, we propose the use of Web Services Conversation Language (WSCL) to express the views of tenant and provider separately. To overcome deficiencies in WCSL for expressing heterogeneous data, process rules, and business rules, we extend the syntax of WSCL. We also put forward a new modeling method for constructing SaaS Service, describing the modeling process and the algorithm for obtaining the tenant model from the business model. In conclusion, we describe the modeling tools and validation methods. Keywords-SaaS...