Large-scale studies, such as the Trends in International Mathematics and Science Study (TIMSS), provide data to understand cross-national differences and similarities. In this study, we aimed to identify factors predicting mathematics achievement of Turkish students by comparing to Australian students. First, construct equivalence and item bias were evaluated to check the comparability. Second, factors predicting mathematics achievement of Turkish and Australian students were identified. Then, propensity score matching on background variables was conducted to identify the remaining achievement differences. Results indicated that mathematics skills were free of construct bias in these groups. After removal of some biased items, we obtained an item bias free booklet. Additionally, students' self-confidence and educational resources at home were significant predictors of achievement. Propensity score analysis indicated that educational resources and to a somewhat lesser extent self-confidence were effective in explaining achievement differences between these two countries.