Reports from the Trends in International Mathematics and Science Study (TIMSS) consistently show that there is a substantial gap in average mathematics achievement between Singapore and the USA. This study conducts an exploratory comparative investigation on the multilevel factors influencing the mathematics achievement of students from these two countries. A conceptual framework together with an integrated model is proposed for the analysis. The TIMSS 2011 eighth grader mathematics data were chosen for this study. Results reveal that the common features influencing mathematics achievement for the students of both countries are student self-confidence, school composition by student background and teacher confidence in teaching mathematics. The cause of the different effects of American students' mathematics achievement is more associated with resources, whereas the effects of Singaporean students' mathematics achievement are more related to attitudes, expectations and motivations. The major elements that affect Singaporean student mathematics performances are at the teacher level. Suggestions for future research are discussed.