We examined the influence of several students' cognitive and motivational factors on 8th graders' science achievement and also gender differences on factors that significantly contribute to the science achievement model. A total of 99 girls and 83 boys responded all the instruments used in this study. Results showed that girls outperformed boys on science achievement. Furthermore, regression analyses showed a model including initial conceptual knowledge, scientific reasoning, and utility value of science as independent variables best predicted science achievement. Results also showed girls and boys did not differ on initial conceptual knowledge and scientific reasoning but on utility value of science. Implications for science education were discussed according to the findings.