The significance of free living heterotrophic flagellates in the microbial food web as the main consumers of bacteria and picophytoplankton has now been broadly accepted. However, heterotrophic flagellates have been largely overlooked in limnological studies, mainly because of methodological and taxonomic problems. During the period from April 2010 to March 2011, the planktonic heterotrophic flagellate community parameters, namely abundance, biomass, size distribution, and taxonomic composition were investigated monthly using a live-counting technique. Surface water samples were taken from three sampling sites in Baiyangdian Lake, a typical large shallow fresh water lake in north China. In order to determine the factors related to heterotrophic flagellate communities, twelve environmental factors (including physico-chemical index and biotic parameters), namely pH, water temperature (℃), electrical conductivity, dissolved oxygen, total phosphorus, total nitrogen, ammonia nitrogen, nitrate nitrogen, chlorophyll-a concentration, the abundance of bacteria, planktonic algae and ciliate were also measured. The relationships between the heterotrophic flagellate community parameters and the environmental factors were also analyzed. Results showed that the water temperature and pH of the three sampling sites varied from 0.2 ℃ to 34.2 ℃ and varied from 7.7 to 9.3 respectively; the annual mean concentration of total phosphorus, total nitrogen, ammonia nitrogen and nitrate nitrogen were in the range of 0.20 1.23 mg/L, 1.70-28.91 mg/L, 0.65-21.63 mg/L and 0.63-2.61 mg/L, respectively; the annual mean concentration of chlorophyll-a varied from 1.4 to119.3 μg/L; the annual mean abundance of bacteria, planktonic algae and ciliate were in the range of (1.998-4.827)×10⁶ cells/mL, 316-9782 ind./mL, and 1-238 ind./mL, respectively; the abundance and biomass of heterotrophic flagellates in surface water of Baiyangdian Lake were in the range of 100-3200 ind./ml and 35.98-2328.85 μg/L, and the annual mean abundance and biomass of heterotrophic flagellates were 1054 ind./ml and 532.93 μg/L, respectively. Some species, such as Cyathomonas truncata, Paraphysomonas vestita, Salpingoeca sphaericola, Bodo spp., Rhynchomonas nasuta, Monas spp., and Petalomonas pusilla were dominant species in Baiyangdian Lake during sampling periods. Heterotrophic flagellates in a smaller size (＜7 μm in length) accounted for 47.11% of the total abundance and accounted for 6.82% of total biomass; Heterotrophic flagellates in a middle size (between 7 μm and 14 μm in length) accounted for 37.42% of the total abundance and accounted for 39.21% of the total biomass; heterotrophic flagellates in a larger size (＜14 μm in length) accounted for 15.47% of the total abundance but contributed 53.97% of the total biomass. Statistical analysis indicated that the abundance of heterotrophic flagellates is significantly and negatively correlated with total nitrogen (TN) and ammonia nitrogen, but positively correlated with pH,bacterial abundance, ciliate abundance and chlorophyll a concentrations. However, the biomass of heterotrophic flagellates was positively correlated with pH, dissolved oxygen (DO), ciliate abundance and chlorophyll a concentrations, but negatively correlated with total nitrogen (TN) and ammonia nitrogen. The data obtained in this study demonstrate that heterotrophic flagellate is not only sensitive to environmental factors, but also a key player in the trophic interaction between bacteria and higher trophic levels.