This paper proposes a model and architecture of service-oriented situational method engineering. The model and the architecture are designed as part of service-oriented situational method engineering (SME) framework, which is proposed as a solution to increase SME application on a software project. By applying SME, a project-specific or situational software development method can be composed from several method chunks. Currently, method engineers maintain their own method chunks in a method base built for a specific CAME tool. It would be better if the concept of service-oriented is applied to provide sharing mechanism of method chunks between method engineers. The SOSME model consists of three participants adopted from Service-oriented Architecture (SOA) model: method chunk provider (MCP), method chunk registry (MCR), and method chunk client (MCC). MCPs publish their method chunks to the MCR. Method engineers can build a new (situational) method by using a CAME tool as the MCC which retrieves a list of suitable method chunks from the MCR. Once the method chunks are found, the MCC can retrieve method chunks description from the MCP. The proposed model also includes method chunk representation and searching mechanism from the MCR, method chunk representation and retrieving mechanism from the MCP, as well as method construction and method enactment mechanism at the MCC. The proposed architecture is designed based on the proposed model. The proposed architecture consists of three components: Method Base Management System (MBMS) as part of the MCP, UDDI-like MCR, and Computer Aided Method Engineering (CAME) as a Service (CMaaS) and Computer Aided Software Engineering (CASE) as a Service (CSaaS) as parts of the MCC. Each component is designed based on an existing CAME and CASE architecture, which is modified to apply SOA and Software as a Service (SaaS) concepts.