Abstract: To study the physiological effects and mechanisms of mepiquat chloride (N ,N-dimethyl piperidinium chloride) on the chilling resistance of sweet pepper(Capsicum annuum),the effects of mepiquat chloride on the lipid peroxidation,electrolyte leakage,reactive oxygen species,hormones balance and antioxidant enzyme activities of sweet pepper seedlings were examined under cold stress.Sweet pepper seedlings were irrigated with 10 mL/plant of 50,175 and 500 mg/L mepiquat chloride and planted under 10-15 ℃ for 30 d,and then cold stressed under 5 ℃ for 48 h.The results showed that the growth of seedlings were restrained by mepiquat chloride,and the development of lateral root was promoted.The concent of ABA,proline,the efficiency of H2O2 eliminating were increased in the seedlings treated with different mepiquat chloride concentration,and the concent of malondialdehyde (MDA) was reduced,which consequently relieved the damage of the membrane.Under cold stress,the activities of antioxidant enzymes superoxide dismutase (SOD,EC 1.15.1.1),catalase (CAT,EC 1.11.1.6) in leaves were increased.In conclusion,mepiquat chloride,between 175 mg/L and 500 mg/L,could induce the chilling resistance of sweet pepper seedlings.