This cross-cultural study examines national variations in the association of youths' attitudinal characteristics and socio-psychological resources at school with their mathematics achievements between South Korea and the US. Using the Trends in International Mathematics and Science Study 2015 data, it finds comparative results between two countries. By employing multilevel modeling, we discover that youths' attitudinal factors such as educational aspirations, interest, and values are closely associated with mathematics achievement, but the association is stronger in South Korea. Also, teacher effect is more prominent in the US while peer effect is more noticeable in South Korea. These results imply that socio-psychological resources are accumulated differently in the two countries, and shared educational aspirations would be an important factor for Korean youths in collectivism and Confucianism society.