In recent years, studies on the size variation of organism have become an important component of life history strategy research. In this study, the significant body size variations of different Brachionus calyciflorus clones hatched by resting eggs produced by different mothers with the same body size were found, and the largest body size was 6.25 times that of the smallest one. It is hypothesized that producing offspring with different body sizes in rotifer population is an adaptive evolutionary strategy to cope with environmental changes. However, few attempts have been made to investigate the genetic and life-history characteristics of those large and small rotifer clones. Hereby, in the present study, the genetic differentiation and classification status were compared between large and small B. calyciflorus clones by molecular markers of mtDNA COI genes and rDNA ITS sequences, and their life history characteristics were also examined at 20℃, 25℃,30℃ and 1.0×10⁶, 3.0×10⁶, 5.0×10⁶cells/mL food densities of Scenedesmus obliquus. The results showed that body size variations in B. calyciflorus populationwere not attributed to a marked divergence of genetic traits. Large and small rotifer clones showed no differentiation of sibling species by the two molecular markers, and two morphs had the shared haplotypes. Temperature, food density, rotifer morphotype, and the interactions between temperature or food density and rotifer morphotype all significantly affected life history characteristics of B. calyciflorus. At the food density of 1.0×10⁶ cells/mL, small rotifers displayed longer embryo and juvenile periods and shorter reproduction period markedly. Large rotifers did not reveal significant changes of embryo and reproduction periods in response to food level variation, but showed longer juvenile period at 1.0×10⁶ and 3.0×10⁶ cells/mL food densities. Compared to small rotifers, the reproduction period, average lifespan and generation time of large rotifers became longer significantly, or tended to be longer significantly at each temperature and food density. The difference in population growth between the two morphs differed with temperature and food density. The reproductive capacities of the two morphs were similar in treatments with 20℃, 25℃, 3.0×10⁶, and 5.0×10⁶ cells/mL food densities, but that of small and large rotifers was stronger at 30℃ and 1.0×10⁶ cells/mL food density, respectively. Small rotifers did not have mictic female offspring at each temperature and food density, while large rotifers had higher proportion of mictic offspring at 20℃. Overall, large and small B. calyciflorus clones had different life history strategies, and significant body size variations in offspring produced by sexual reproduction could occur by the evolution of bet-hedging strategies to adapt to environmental unpredictability.