A critical incident is an unexpected event that occurs in the classroom. Teachers can uncover a new understanding of the teaching and learning process through reflecting on these critical incidents. This study explored the writing and discussion of critical incidents among six student teachers with their cooperating teachers in a practicum under the elementary school language teacher education program in a university in northwest Taiwan. The researcher used the conceptual framework designed based on Vygotskij's sociocultural theory of learning to analyze the data. Critical incidents were used as tool-mediated activities. The analysis of critical incidents, observation notes, and interview transcripts revealed that student teachers gained professional learning in terms of individual learners' differences, classroom management, instructional strategies, solutions to incidents, and reflections through discussing with student teachers and cooperating teachers. Three suggestions were provided in order to effectively integrate writing down and discussing critical incidents into practicum.