A web service usually serves many consumers, with each consumer having its specific functional or nonfunctional requirements. The web service should be customizable to meet those requirements, as is especially apparent for complex web services with large granularity such as Softwareas-Service (SaS). This paper proposes an approach to facilitating web service customization in the programatical way through the collaboration between service provider and consumer. A specification for declaring web service customization policy is defined based on the WS-Policy framework, through which service provider can declare the service's customization capabilities. A service consumer can make customization requests within the scope defined by the customization policies. The customization requests are raised in structuralized customization directives. A web service consumption process and a supporting framework accompanied with customization policy are introduced accordingly. Finally, a case study is present.