Abstract: The phenotypic composition of groups affect ecological and social processes, which is crucial for collective behavior research. To explore the influence of body size variation on shoal behavior between two cyprinids with diffe-rent habitats, the juvenile Chinese bream (Parabramis pekinensis) preferring still water and Qingbo (Spinibarbus sinensis) enjoying flowing water were selected to study spontaneous swimming behavior, spatial structure and the response to startling stimulus of fish groups including four similar fish individuals or including two larger and two smaller fish individuals. The results showed that the spontaneous swimming speed, the synchronization of speed and the alignment polarity of the juvenile Qingbo were significantly higher than those of Chinese bream, and that the responses to frigh-ten stimulus were similar between two species. The polarities of equal-size groups were similar to those of non-equal groups while the non-equal groups showed lower synchronization of speed in both species. However, all measured variables were not different between large and small size individuals within non-equal size group. The difference in body size did not impact group cohesion. The larger individuals of heterogeneity in spatial structure preferred to stay at the front of fish shoal. The flow regime habitats of Qingbo may explain its higher activeness and synchronization of swimming speed and alignment polarity than Chinese bream. Group members with different body size can coordinate their swimming speed and direction of movements even with higher group cohesion. Interestingly, smaller Qingbo indivi-duals elevated their speed to keep pace with larger individuals whereas Chinese bream individuals adjusted their speed to a moderate value. Variation of body size decreased the response ratio to frighten stimulus, possibly due to decreased response acuity to external stimulus as a consequences of social factor (e.g. social hierarchy), and impaired efficiency of information flow and (or) the divergence of ecological benefits and costs between larger and smaller size individuals, which subsequently influenced the consensus decision making.