This paper reported using the practical inquiry model as discourse guide to facilitate students' critical thinking in online discussion. It was found that almost all the postings of the students who had no knowledge of the inquiry model fell into exploration phase except three postings in triggering events phase and two in integration phase. In comparison, the postings of the students who used the model as the guide included more instances of integration than the postings of those who did not know about the model. No instance in resolution phase was found. The findings indicated that providing students inquiry model raised their awareness of critical thinking and helped them intentionally engage in reflection and higher-order thinking when responding online. (Contains 2 tables.)