This study is to develop model for collaborative digital reference service which transfer questions and answers between collaborative libraries via formal protocol. For collaboration, interoperability of reference transaction must be needed. Therefore, for seamless and systematic collaborative digital reference service, this model has shown transaction for processing of questions and answers, and standard formats for exchange and knowledge base of question and answer pairs, with collaboratively building internet reference resources and sharing reference database. This model will enhance digital reference service locally and networked digital reference.