Abstract: Global warming affects the survival, reproduction, collective behavior, and population dynamics of the fish. To examine thermal tolerance and collective behavior during the heating in the Cyprinidae fish, our study selected the juvenile grass carp (Ctenopharyngodon idellus) as the experimental fish. The thermal tolerance, individual swimming speed, synchronization of speed, inter-individual distance, and other behavioral parameters were obtained in both the large and small body size juvenile grass carp groups. Compared with the small body size group, the large body size group has higher swimming speed (P=0.039), higher inter-individual distance (P<0.001), higher nearest neighbor distance (P<0.001), and higher critical thermal maximum (P<0.001). There is no difference in the group polarization and lethal thermal maximum between the two groups. During the process of water temperature rising, individual swimming speed of the juvenile grass carp increased profoundly, while the inter-individual distance and nearest neighbor distance are also increased, but the synchronization of speed and the group polarization remained unchanged. These results showed that increase of individual body size can enhance the thermal tolerance of the juvenile grass carp. Water temperature rising do not affect the coordination of the juvenile grass carp group, but reduce the cohesion of the juvenile grass carp group.