Probability is an important mathematics topic and is required for all Thai secondary school students. However, previous research found that both students and teachers often held several probability misconceptions, students had difficulties learning the topic, and teachers did not have sufficient knowledge to teach the topics. The purpose of this study was to investigate the types of probability misconceptions held by Thai secondary school students and their mathematics teachers, how the teachers implemented a nationwide inquiry-based curriculum, and how instruction impacted the students' understanding of the concepts. The participants were two grade 9 mathematics teachers and their students in four classes, 204 students in total. The data collection included lesson observations, a probability misconception pretest and posttest, and teacher interviews. The results showed that both the teachers and the students held several types of probability misconceptions. Even though the students' performance on the posttest was significantly improved after instruction, they still did not do well. The students learned knowledge on probability as evidenced in their responses on the posttest, but they were still having trouble appropriately applying the knowledge. Teachers' probability misconceptions and their understanding of probability topics affected how their students learned and understood probability.