Software as Services (SaS) is emerging as a new application software delivery approach, and has extensively leveraged Web services and other SOA concepts and technologies. Compared to the traditional ones, SaS has the following distinctive features: application delivery over Internet, usage based pricing mechanism, etc. Potentially, this approach can significantly reduce the total cost of ownership of application software. However, it does have a number of unique issues that include service customization, multi-tenancy, complex billing model, etc. If not handled properly, these issues can result in ad-hoc and manual service consumption practices, which could eliminate the benefits of the SaS approach. In this paper, we are going to specifically deal with the service customization issue. To that end, we proposed a novel SaS customization policy and a supporting framework that is realized through a design-time tooling and a runtime environment. We also provided a case study to verify the effectiveness of the proposed approach.