The mass flows and removal of 20 antibiotics of seven classes in two wastewater treatment plants (WWTPs) of Hong Kong were investigated in different seasons of a whole year, using bihourly 24h flow proportional composite samples. Antibiotics were detected at concentrations of 3.2–1718, 1.3–1176 and 1.1–233ngL −1 in influents, secondary and disinfection effluents. Total daily discharges of all the detected antibiotics from effluents of Shatin and Stanley WWTPs were 470–710 and 3.0–5.2gd −1 , respectively. Ampicillin, cefalexin, sulfamethoxazole, sulfadiazine, sulfamethazine, chlortetracycline and vancomycin were effectively (52–100%) eliminated by activated sludge process while ampicillin and cefalexin were effectively (91–99%) eliminated by disinfection. Bihourly variation analysis showed that concentrations of the major antibiotics in influents varied more significantly in Stanley WWTP which served small communities.