Characteristics of seed sludge are of great importance in granulation and treatment performance of aerobic granular sludge process. Fast-settling floccular sludge has been shown to be affective on accelerating granulation. High-rate activated sludge process provides organic matter capturing from municipal wastewaters in order to harvest the chemical energy through adsorption rather than mineralization. Thus, waste sludge from the highrate activated sludge process settles fast since the biosorption is the main mechanism in this technology. In this paper, impact of different seed sludge characteristics on granulation, and treatment performance of an aerobic granular sludge system was comparatively evaluated by seeding the system with solely conventional waste activated sludge (Stage 1) and a mixture of waste sludge from a conventional activated sludge process and a high-rate activated sludge process (Stage 2). Although the aerobic granular sludge reactor was seeded with a sludge that had a higher median particle size at Stage 2, after steady state conditions have been reached during the operation, average median particle size in the reactor at Stage 1 (124 +/- 2.8 mu m) was higher than that at Stage 2 (86 +/- 2.7 mu m). High ammonium nitrogen removal efficiencies (>98%) obtained in the study, however, total nitrogen (TN) and total phosphorus (TP) removal efficiencies obtained at Stage 2 (TN: 66.7 +/- 2.2%; TP: 13.2 +/- 2.9%) were lower those at Stage 1 (TN: 80.9 +/- 2.2%; TP: 92.7 +/- 4.4%). One of the reasons of incomplete denitrification and decreased phosphorus removal efficiencies observed at Stage 2 was smaller granules that was the cause of less anoxic/anaerobic volume. Another possible reason was the absence of denitrifiers, and polyphosphate-accumulating organisms in the high-rate activated sludge. Although high efficiencies were achieved in COD and ammonia removal by aerobic granular sludge process seeded with different types of sludge, this study showed that seed sludge affected anoxic/anaerobic volumes in the granules and thus affected denitrification and phosphate accumulation. Nitrogen and phosphorus loads resulting from wastewater treatment plants must be controlled and decreased before their discharge into the receiving water bodies. From this perspective, this study highlights the importance of seed sludge type on nitrogen and phosphorus removal performance of aerobic granular sludge process.