Objective To investigate the effects of thrombolytic therapy on recanalization in different starting time of treatment after acute myocardial infarction. Methods 172 cases with intravenous thrombolytic agents were divided into 4 groups: less than or equal to 2 hours, > 2-4 hours, > 4-6 hours and > 6-12 hours, according to the different starting time of therapy after onset of symptoms. Results The recanalization rates were 78.6%, 74%, 39.6%, and 14.3% in less than or equal to 2 hours, > 2-4 hours, > 4-6 hours and > 6-12 hours, respectively. The recanalization rate within 4 hours was significantly higher than those in the groups of > 4-6 hours AND > 6 similar to 12 hours. The time interval from the initiation of thrombolytic therapy to reperfusion was increased as the starting lime of thrombolysis after onset of symptoms was delayed. Conclusions Starting time of thrombolysis within 4 hours after onset of acute myocardial infarction is of the best effect on recanalization.