The aim of this study was to examine the association between students' TIMSS (Trends in Mathematics and Science Study) science achievement, and students' school achievements, in terms of national tests and grades from school years 6 and 9. Further to examine the association with TIMSS science achievement and different subgroups of students based on their home background. The study is based on a unique possibility to analyse TIMSS 2015 data together with register data of the Swedish students' national test results from school years 6 and 9 and their science subject grades from school years 6 and 9. The overall results show that there were moderate associations between TIMSS science achievement and school achievement measures. The association between grades and the high-stakes national tests were stronger than between grades and TIMSS. The students' home background had a clear impact on the results as students with highly educated mothers, who comes from homes with many books and are nonimmigrants had on average higher TIMSS science achievements.