Problem Based Learning (PBL) is proposed as one of the most well-known alternatives to traditional lectures in modeling engineering graduates to become independent workers, critical thinkers, problem solver, lifelong learners, and team worker. In the past decades much has been reported about how to support online PBL, but much less attention has been given to support assessment in an online PBL. Recently, the authors of this paper have developed a domain-specific modeling language approach and an associated web-based PBL application to support the development and delivery of online PBL modules in a cost-effective and flexible manner. In this paper, the authors emphasize to support peer assessment in an online PBL. Through using a peer assessment example, the authors present how to support teachers, who may not have a comprehensive technical knowledge, to specify a peer assessment process as a segment of a PBL script, a graphical representation of a PBL strategy. Then the PBL application can transform a PBL script that including the peer assessment into a formal model represented in an international e-learning technical standard IMS-Learning Design (IMS-LD). Finally, such a formal model can be executed in any IMS-LD compatible run-time environment. We have demonstrated the technical feasibility of this approach through specifying and transforming the example peer assessment script.