The purposes of this study were to explore how a science teacher and a mathematics teacher collaborated for science and mathematics integration with the support of a researcher, to describe the process of planning and implementing integration and to understand the main issues considered by teachers during the process. Observational case study was employed for the study. One mathematics teacher and one science teacher working at the same school volunteered to collaborate for integration. Three science-based and two mathematics-based integrated plans were prepared by the team and implemented by the teachers. The analysis of the planning and implementation process of integrated plans showed that (i) determining objectives, (ii) checking for students’ prerequisite knowledge and (iii) integration for different purposes were important for teachers. There is a need to explore teachers’ collaboration for integration in different contexts and the effects of such collaboration on students’ learning.