In this study, it is aimed to examine the effect of classroom assessment on science and mathematics achievements. For this purpose, hierarchical linear modeling (HLM) is performed using variables of like learning science/maths, engage teaching in science/maths, confidence in science/maths, and home resources for learning variables at the student level, and experience, education level, homework, and assessment at the teacher level. The sample of the study consists of 4th grade students who participated in TIMSS 2015 in Turkey. According to the findings; 36% of variance in science achievement, and 40% of variance in mathematics achievement are due to variability between classes. In a random coefficient model, all student variables were found to be statistically significant predictors of science and mathematics achievement. Among these variables, the greatest effect size is self-confidence variability. Only the teacher variables are added according to the Means as the outcome model; the teacher's experience and emphasis to national achievement tests of monitoring students' progress had a statistically significant effect on science and mathematics achievement. Finally, according to the intercept and slopes of the outcomes model, the most important variable is the emphasis to national achievement tests of monitoring students' progress in both science and mathematics.