Competition for food resource is one of the crucial biological interactions in nature. According to the size-efficiency hypothesis, large filter-feeding cladocerans are viewed to be superior to small cladocerans. However, large filter-feeding cladocerans generally tend to have higher phosphorus content than small cladocerans, and food stoichiometry quality may affect the outcome of competition among them. We examined the effects of food stoichiometry quality (different carbon phosphorus ratios (C:P)) and interspecific competition on the population growth of two common cladocerans species in South China, Daphnia galeata and Bosmina fatalis. D. galeata and B. fatalis were fed saturating concentrations of food (Chlorella pyrenoidosa, 2 mg C/L) of high P (C:P=105), medium P (C:P=740), and low P (C:P=2400), and grew alone or together. Peak population densities varied from 378 to 893 ind./L and 364 to 2399 ind./L, and population increase rates varied from 0.11 to 0.14 d^-1 and 0.09 to 0.16 d^-1 for D. galeata and B. fatalis, respectively. Mean body length varied from 1.53 to 2.50 mm for D. galeata, and 0.25 to 0.35 mm for B. fatalis. Rate of population increase and mean body length in both species decreased with the increasing of food C:P ratio, regardless of growing alone or together. However, peak population densities in both species were much higher under medium P condition than those under high and low P conditions, regardless of growing alone or together. In mixed cladoceran cultures, population increase rates of D. galeata were not significantly different from those cultured alone, while increase rates of B. fatalis were much lower than when cultured alone, regardless of the C:P ratio in food. Only under low P condition did the peak densities of D. galeata tended to be much lower in mixed cladoceran cultures than when cultured alone. For B. fatalis, peak population densities were much lower in mixed cultures than when cultured alone in medium and low P conditions. Our results suggest that D. galeata is competitively superior to B. fatalis irrespectively of food C:P ratio when food concentration is high enough for their population growth.