This study utilized data from the 2006 Programme for International Student Assessment Hong Kong sample to investigate the factors that impact the science achievement of 15-year-old students. A multilevel model was used to examine the factors from both student and school perspectives. At the student level, the results indicated that male students, students from high socioeconomic status (SES) families, students with higher motivation and higher self-efficacy, and students whose parents highly value science are more likely to demonstrate achievement in science. At the school level, the results showed school science achievement differences can be explained by school enrolment size, school SES composition, and instruction time per week. Contrary to the negative influence of school size that was reported in previous studies, our findings suggested a positive relationship between school enrolment size and science achievement. This finding leads to an international discussion of school size.[Copyright of International Journal of Science Education is the property of Routledge. Full article may be available at the publisher's website: http://dx.doi.org/10.1080/09500693.2012.708063]