This study investigates Spanish and Chinese students' performance on both addition problems and proportion problems considering a cross-national perspective. The effect of number structure and nature of quantities was also considered. Nine hundred twenty-five 4th to 8th graders (453 Chinese, 472 Spanish) took a test which is composed of addition and proportion problems in a missing-value format. The findings showed that Chinese students held a different developmental model of the additive and proportional reasoning from Spanish students. Moreover, number structure and nature of quantities had a different impact in each country. These results indicated that students' additive and proportional reasoning may vary from country to country due to the different cultures and educational environments. Further studies with a larger-scale sample will be needed to confirm such cross-national effects.