A Web service is a software system designed to support interoperable application-to-application interactions over the Internet. Web services are based on a set of XML standards, such as Web Services Description Language (WSDL), Simple Object Access Protocol (SOAP) and Universal Description, Discovery and Integration (UDDI). Recently, there has been a growing interest in Web service composition, and some languages (e.g., WSBPEL, BPML) for modeling the composition have been proposed. In this paper, we focus on security constraints of Web service composition, which have not been deeply investigated so far. We propose a method for modeling security constraints and a brokered architecture to build composite Web services according to the specified security constraints.