In-service science teachers in Thailand are mandated to conduct classroom research, which can be quantitative and qualitative research, to improve teaching and learning. Comparing to quantitative research, qualitative research is a research approach that most of the Thai science teachers are not familiar with. This situation impedes science teachers' ability to conduct classroom research for answering a wide range of research questions especially qualitative research questions. This study aims to promote the in-service science teachers' understanding of and attitudes to qualitative research. The research's methodology was a case study. The participants were 32 science teachers (6 male, 26 female) from Uthai Thani province, Thailand. They were asked to attend a three-day qualitative classroom action research (QCAR) workshop. The data collection process included the survey of problems concerning classroom action research (CAR), the pre- and post-surveys of attitudes to CAR, and the reflection on learning from the QCAR workshop. The data were analyzed both qualitatively and quantitatively. The results revealed that the three most problems concerning CAR for the participants were writing qualitative research report, analyzing qualitative data, and designing qualitative research instruments. After attending the QCAR workshop, the participants gained more understanding of CAR and significantly improved their attitudes to the CAR (t = 4.739, p < 0.01). The findings from this study lead to the improvement of the QCAR workshop for in-service science teachers nationwide.