The purpose of this study was to investigate the contribution of the motivational beliefs (self-efficacy and task-value) and cognitive engagement to seventh grade students' science achievement. For the specified purpose, cross-sectional correlational research design was used. The data were gathered from the seventh grade students of public middle schools by means of three data collection instruments namely, Background Characteristics Survey (BCS), Motivation and Cognitive Engagement Scale (MCES) and Science Achievement Test for 7th Grade (SAT). The MCES is a self-report instrument including the selected items from the Science Learning Inventory (SLI- Part A) and from Turkish Version of Motivated Strategies for Learning Questionnaire (MSLQ) in order to measure students' motivational beliefs (self-efficacy and task-value) and the level of their cognitive engagement. A total of 861 seventh grade students (398 girls and 456 boys) participated in the study. Multiple Linear Regression Analysis was used to analyze the data. Results revealed that motivational beliefs (i.e. self-efficacy and task value) positively and significantly contributed to the prediction of students' science achievement and the self-efficacy appeared as the best predictor of the science achievement. Cognitive engagement failed to significantly predict students' science achievement. Finally, bivariate relations among independent variables (self-efficacy, task-value and cognitive engagement) were examined through simple correlation analyses. The result indicated positive and significant correlations among self-efficacy, task-value and cognitive engagement variables.