This research compared the relationships between achievement motivation, self concept, and achievement in science among students of 38 countries, as presented in the Third International Studies of Science and Math (TIMSS-R). The sample was 187000 eighth graders of the countries that participated in the 1999 TIMSS test, randomly selected by the International Institute for Academic Achievement Evaluation. The material included a Science Questionnaire, consisting of 146 items, and an individual questionnaire and both were administered by the ministries of education of the related countries. The results were published in year 2000 and later used in this study. The results of ANOVA, Kruskal-Wallis, and inde- pendent t-test showed a significant difference between students' self concept, expectations about continuing their education, and academic achievement. But, there was no significant difference between the students' attitudes, interests, perceptions regarding the importance of science, time spent out of home to learn the material, gender, and achievement in science.