In this study, suspended growth reactors (SGRs) were used to evaluate the effect of floc particle size on nitrification and assimilation capacity. In bioflocculation, there are flocs of various sizes; the size of the bioflocculation is affected by the aeration intensity and the osmotic pressure and the size of the floc is adjusted to achieve the strongest water treatment capacity. Therefore, the differences between flocs of different sizes and their role in water treatment are worth exploring. The purpose of this study was to investigate the effect of floc particle size on assimilation, nitrification, and nutrient values. Three treatment groups were set up; the <50 μm particle size group, the >50 μm particle size group, and the unscreened group. Under nitrification conditions, the total ammonia nitrogen (TAN) removal rates of the unscreened group, >50 μm group, and <50 μm group were (1.33±0.01) mg TAN/(g TSS·h), (1.62±0.04) mg TAN/(g TSS·h), and (1.64±0.06) mg TAN/(g TSS·h), respectively; under assimilation conditions, the TAN removal rates of the three groups were (2.83±0.08) mg TAN/(g TSS·h), (3.34±0.12) mg TAN/(g TSS·h), and (3.52±0.12) mg TAN/(g TSS·h), respectively. There were no significant differences in the TAN removal rates, nitrite nitrogen (NO₂^--N), nitrate nitrogen (NO₃^--N), and total nitrogen (TN) between the >50 μm and the <50 μm group (>0.05). We also measured the dissolved organic carbon (DOC), crude protein, crude fat, amino acids, fatty acids, crude ash, carbon to nitrogen ratio (C/N), volatile suspended solids (VSS), specific oxygen uptake rate (SOUR), and other indicators. The total fat content was higher in the >50 μm group and the crude protein content was higher in the <50 μm group. Our results showed that the floc particle size had no significant effect on the nitrification and assimilation reactions; under different reaction conditions, the floc particle size affected the nutritional value of bioflocculation and it also affected greatly the microbial community composition. Our results also showed that sieving might have had some effect on the flocs. This effect was greater than the influence of the different floc particle sizes. Sieving may have enhanced the microbial activity in or changed the morphology of bioflocculation. However, structural changes in bioflocculation require further verification. We conclude that the particle size had no significant effect on the nitrification and assimilation reactions and had a significant effect on the nutritional value of bioflocculation. This study has the potential to effect changes on the management of biological flocculation systems. The effects of floc size on microbial composition, nitrogen conversion, and nutrient composition should be studied further.