In this study, we developed an instrument for assessing teachers' mathematics content knowledge (MCK) on ratio and proportion and examined the profile of Indonesian primary teacher's MCK on this topic. The MCK items were administered to 271 Indonesian in-service primary teachers with a variety of educational backgrounds and teaching experiences. Teachers' responses were analyzed by factor analysis and cluster analysis. The MCK instrument was found to have good acceptability in the reliability analysis with 3 factor components--"meaning of proportional and non-proportional situations," "number structures in situation," and "figural representation"--which was the main result of the study. With respect to the 3 factors, the teachers in the 3 assigned categories ("Good," "Middle," or "Low") showed consistent performance on the items of the 3 factors. In particular, our results indicated that Indonesian in-service primary teachers had difficulty with the factor on figural representation, but they performed best on number structures in situation representing products of proportional reasoning.