This research purposes to promote a Gases Theory Representation Instrument (GTRI) as a tool to identify the students’ conception on kinetic theory of gases. The method used in this research was FODEM (Formative Development Methods) model which has three comprehensive steps, which are need analysis, implementation, and formative evaluation. The participants involved in this research were 26 high school students in Sundanese tribe. The students’ responses were analyzed using Rasch model, which involved item reliability, person reliability, validity, difficulty level and students’ conception distributions. Students’ conception were classified into six categories which are Sound Understanding (SU), Partial Positive (PP), Partial Negative (PN), Misconception (MC), No Understanding (NU), and No Coding (NC). Based on the data analysis, it can be concluded that students’ conception are typically in the SU and PP categories. Besides, the Gases Theory Representation Instrument (GTRI) is reliable and valid to identify students’ conception on kinetic theory of gases.