The purpose of this study was to explore primary grade teachers‘ stages of concern and their implementation of an educational reform initiative called Integrated Thematic Instruction (ITI) in Indonesia. The Indonesian minister of education mandated primary grade teachers to implement ITI in 2006. Using a convenience sampling method, 150 Kanisius primary grade teachers participated in the study. The data collection method used was a questionnaire that consisted of three parts: The Stages of Concern Questionnaire (SoCQ), the Integrated Curriculum Implementation Scale, and demographic questions. The data analysis employed in this research were descriptive, a one way ANOVA test and multiple regression. The result of the study revealed that the teachers have unresolved awareness and self-informational concerns with a slight tailing up in the impact-refocusing stage. This study also found that teachers‘ degree of ITI implementation was high some five years into this reform initiative. Additionally, there were no differences among the different groups with different attitudes and behaviors related to ITI implementation as measured by the SoC. Administrative support was found to be the most important predictor of the ITI implementation followed by the number of students taught and educational degree. An increase in respondents‘ perception of having administrative support and their educational background increased the degree of ITI implementation, while an increase in the number of students lowered the degree of ITI implementation. Recommendations of this study include the need for administrative support to stimulate the interest of teachers in using ITI, to facilitate a climate conducive to continued use of ITI, to provide the most appropriate type of professional development for teachers, as well as to build a comprehensive system for monitoring, supervising, and scaffolding teachers‘ implementation of ITI.