We aimed to compare the therapeutic effect of recombinant tissue plasminogen activator (rt-PA) administered at different time windows within the first 6 hours after onset of acute ischemic stroke (AIS). A retrospective analysis was performed of data collected from 194 patients who received rt-PA thrombolysis within 4.5 hours after AIS onset and from 29 patients who received rt-PA thrombolysis between 4.5-6 hours after AIS onset. The National Institutes of Health Stroke Scale (NIHSS) scores were statistically decreased in both groups (P < 0.05) at 24 hours and 7 days after onset. There was no statistical difference in the modified Rankin score or mortality at day 90 after treatment between the two groups (P > 0.05). In conclusion, AIS patients who received rt-PA treatment between 4.5-6 hours after onset were similar in therapeutic efficacy to those who received rt-PA within 4.5 hours after onset. Our results suggest that intravenous thrombolytic therapy for AIS within 4.5-6 hours after onset is effective and safe.