Big-Fish-Little-Pond-Effect (BFLPE) suggests that students form their own self-concept by comparing their abilities to those of their peers. This study aims to examine how students' gender, socioeconomic status, science achievement, and engagement dimensions (motivational and behavioral engagement) predict students' self-concept in science by considering BFLPE. This study used TIMSS 2011 data set including 6928 eighth grade Turkish students from 239 schools. Since the data were in a nested structure (students nested in classrooms) HLM analysis was preferred to minimize the dependences of the data. HLM analyses showed that about 10% of the variance in students' self-concept in science learning was between classes while the rest of the variance was within classes. Findings suggest that students' motivational and behavioral engagements are important indicators of students' self-concept as well as science achievement. Additionally, at the class level, aggregated science achievement was found to be significant and a negative predictor of self-concept, which supported BFLPE.