Mashup is a hallmark of Web 2.0 and attracts both industry and academia recently. It refers to an ad hoc composition technology of Web applications that allows users to draw upon content retrieved from external data sources to create entirely new services. Compared to traditional "developer-centric" composition technologies, e.g., BPEL and WSCI, mashup provides a flexible and easy-of-use way for service composition on web. It makes the consumers free to compose services as they wish as well as simplifies the composition task. This paper makes two contributions. Firstly, we propose the mashup architecture, extend current SOA model with mashup and analyze how it facilitates service composition. Secondly, we propose a mashup component model to help developers leverage to create their own composite services. A case study is given to illustrate how to do service composition by mashup. This paper also discusses about some interesting topics about mashup.