Description: The article deals with Basic School student's mathematics achievement change through 1995-2003 in Lithuania. For the analysis data from TIMSS 1995, 1999 and 2003 is used. The cycles of TIMSS and the scaling methodology used for calculating the scores provide the possibility for participating countries not only to compare their results with the ones from other countries, but also to track the changes in their students' achievements throughout the year. It is of particular importance to those countries that are experiencing considerable changes in their educational systems. Lithuania is one of such countries as it was undergoing a large educational reform since 1990's. Participation in the three TIMSS cycles which had their main surveys in 1995, 1999, and 2003, gave a reliable source to measure the impact of the reform and the general changes in the country's life to the Basic School (Grade 8) students' achievements in mathematics. The question is analyzed using classical statistical investigation (main statistical software used was SPSS 12.0). Some main findings related to the changes in students' mathematics results' will be presented in the conference.