Starch/acrylamide/allyl triphenyl phosphonium bromide (St/AM/TP) copolymers were synthesized by simultaneous gamma irradiation and characterized by FTIR and 1H NMR techniques, weight measurement and titration method. Moreover, their antibacterial activities against Staphylococcus aureus were explored by the viable cell counting method in sterile distilled water. At St/AM/TP 6:8.4:5.6g, copolymers with higher graft ratio (G) and higher (AM+TP) graft efficiency (EAM+TP) were obtained at 3 and 6kGy, while cationic degree (CD) and TP graft efficiency (ETP) continuously increased with absorbed dose from 1 to 6kGy. All of the copolymers were capable of killing >99.75% of 107CFU/ml S. aureus within 30mins. Moreover, at 3kGy, G, EAM+TP and ETP increased with AM/TP from 0:14 to 11.2:2.8g at St/(AM+TP) 6:14g, while the optimum CD and antibacterial activity were achieved at AM/TP 7:7 and 8.4:5.6g. In addition, at 3kGy, G, EAM+TP and CD increased with St/(AM+TP) from 6:3 to 6:18g at AM/TP 8.4:5.6g, while the optimum antibacterial activity was achieved at 6:10 to 6:18g, and the optimum ETP was achieved at 6:14.