This study aimed to improve science teaching in a practical way by activating teachers' potential teaching expertise or professional knowledge. We developed an alternative in-service approach, the Practical On-site Cooperation Model (POCoM), according to the following principles: (1) the "bottom-up" approach, where observed problems in science classes are improved practically and immediately without any pre-determined teaching plan or materials; (2) "cooperation" between researchers and teachers to improve teaching; (3) "naturalistic settings" where various real problems occur in actual teaching and learning situations; and (4) "gradual improvement" rather than revolutionary change. The POCoM was applied with three science teachers, and 24 classes were observed. We found about 65%-96% teaching improvement. The analysis of the cooperation processes and science teaching in classrooms identified seven types of successful improvements and five types of unsatisfactory cases, including their reasons and features.