International comparative research in general and TIMSS research in particular struggle to explain differences in student performance across countries, as well as changes in student performance over years. Clearly, these changes are not caused by one factor, but a set of several factors. This study aims to determine the influence of country level variables on science achievement in TIMSS. As TIMSS has been conducted since 1995, it is possible to obtain trend data on student achievement in science. As a data analysis method panel data regression has been applied. This method is appropriate when the researcher has available observations which are both cross-sectional and time series. This method allows to respect investments that were made in previous years but actually give effect after few years. As data source, the student achievement data from TIMSS 1995–2007 are used as well as annual data on educational systems from a public data basis. It is expected to determine country level factors which affect students’ science achievements in the TIMSS study cycles.