This study examined the preservice biology teachers’ reflective thinking on their science teaching practices during the field experience. The preservice biology teachers learned about the core reflection before the field experience, and wrote reflection papers about their science teaching experiences based on the core reflection. The preservice teachers were asked to think about the problematic situations on their science teaching, to be aware of ideal science teaching and actual situations, to be aware of core qualities on science teaching and actualization of core qualities. The results indicated that the preservice teachers experienced the problematic situations of their science teaching during the field experience when the students did not concentrate or did not participate on the class anymore. Also, they had difficulties of controlling the class as well as explaining appropriately according to students’ levels. The preservice teachers were aware of the ideal situations as the science classes to seek principles through inquiry by students’ active participations, as well as nurture logical thinking. However, these preservice teachers recognized their limitations as qualified teachers, such as lack of the ability to use appropriate teaching methods, communication ability with students, and questioning skills. In order to actualization of core qualities, the preservice teachers mentioned the importance of trust building with students, inquiry learning environment, questioning skills, and scientific knowledge and inquiry attitude as science teachers.