The aim of this study was to investigate the effects of fill time of influent on the physical characteristics of biological floc for sequencing batch reactor (SBR) treating low C/N ratio wastewater. The fractal dimension and size of floc were measured for characterizing floc. The fractal dimension of floc was analyzed by using Small Angle Laser Light Scattering (SALLS) method. SBR with short fill time showed higher removal efficiencies of COD, BOD, T-N, and T-P than with long fill time. Larger floc size and fractal dimension were generated at SBR with short fill time. Short fill time generated the sludge with better settling and thickening properties. Alternating anaerobic and aerobic condition in the cycle also affected the floc size and fractal dimension. The floc size decreased under anaerobic phase, and floc size increased under aerobic phase. During fill time, the fractal dimension of floc decreased. As the fill stop, the fractal dimension of floc increased. Therefore, the fill time condition more affected the fractal dimension of floc. More efficient nitrification and phosphorus release were observed during a cycle with short fill time operation.