The impact of hydraulic retention time (HRT) in an anaerobic baffled reactor (ABR) on its operation performance and granular sludge characteristics was investigated through both testing its operation and analyzing the particle size distribution (PSD) and fractal dimensions of these sludge granules when treating low-strength wastewater. As HRT was gradually reduced from 24 h to 5 h, ABR had good performance on the organics removal and could reach about 90% for the COD removal, the VFA contents in ABR effluent and their pH values showed opposite trends. Most COD in wastewater was removed in the first two compartments of ABR during the running period of HRT from 24 h to 12 h, after that, the middle three compartments in ABR performed the most removal work. The MISS in latter compartments of ABR increased and the tendency of MLVSS/MISS ratios in the former three compartments were higher than that in the latter two ones appeared more and more remarkable along with the decrease of HRT. At HRT 24 h and 18 h, the granules grew bigger than before start-up,whose one-dimensional fractal dimension (D) decreased and two-dimensional fractal dimension (D2) increased, implying the smoother surface and more compact structure. As the HRT changed to 12 h and 8 h, the increasing D1 and decreasing D2 of granular sludge indicated its rougher surface and looser structure, and the bigger granules were formed with holes in their center. At HRT 5 h, smoother and more compact granules formed with smaller number median diameters of 0.72-0.82 mm than that at HRT 8 h. The looser granules were broken-up and some granules were discharged from ABR under the hydraulic disturbance and screening effect, at the same time, the growth of microorganism and their EPS (extracellular polymer substance) secretion could smooth the surface of the fragmentized granules. The fractal dimensions of granular sludge in the one-dimensional and two-dimensional topological spaces behaved inversely in each compartment of ABR at the same HRT, that is to say, the granular sludge along the ABR sequences became smoother on the surface meanwhile more compact structurally.