Biochemical properties of granules are of vital importance to UASB performance. This study characterized the granules cultivated at different COD/SO42- ratios to elucidate the influence of sulfidogenesis on starch wastewater (1000 mg-COD L-1) biodegradation kinetics and process stability. Suitable sulfate addition enriched granular microecosystems and stimulated the secretion of extracellular substances, facilitating cells cohersion and sludge aggregation. The percentage of granules larger than 2.8mm increased from< 10.0% to 58.8-69.4% with decreasing COD/SSO(4)(2-)ratio from 10 to 2. Starch-fed granules tended to grow flagella-like filaments on the surface. The filaments overwhelmed by hydrophilic biopolymers had high affinity for biogas-bubbles and water-molecules aggravating granule floatation and washout. 16 s rRNA gene analysis revealed that decreasing COD/SO(4)(2-)ratio shifted Syntrophobacterales to Desulfovibrio, which co-worked with Methanosaeta while suppressing Methanobacterium thereby altering starch bioconversion routes. Decrease in Syntrophobacterales caused propionate accumulation and slight process upset.