Aim To investigate the influenece of β receptor blocker on Q T dispersion of old myocardial infarction. Methods Q T dispersion(Q Td),diameter of the left ventricular end diastolic phase(DLVED),ejection fraction (EF),paired ventricular premature complex(paired VPC),non sustained ventricular tachycardia(NSVT)were measured in 42 patients with old myocardial infarction(OMI)and 36 patients with hypertensive heart disease (HHD) as control,to investigate the variance of Q Td,paired VPC,NSVT after and before treatment with β blocker. Results Q Td,DLVED,EF were not significant difference (P>0.05) and paired VPC,NSVT were significant difference (P<0.05)between two groups of patients before treatment,significantly greater in OMI group than in control group.Q Td,paired VPC,NSVT were tended to decrease significantly (P<0.05)in OMI group and no significantly(P>0.05) in control group, after treatment with β recepter blocker than before. Conclusion These results suggest that treatment with β blocker may reduce Q Td in the recovery phase and malignant arrhythmia by reducing electrophysiological instability in old myocardial infarction.