What is known and objective Propofol is a most commonly used anaesthetic drug for conscious sedation in outpatient procedures. Previous studies have shown that propofol may affect ventricular repolarization based on QT and Tp‐e intervals. Frontal QRS‐T angle is a marker of ventricular depolarization and repolarization difference. However, there is no study investigating the effect of propofol on frontal QRS‐T angle. In this study, we aimed to investigate the effect of propofol on frontal QRS‐T angle in patients undergoing colonoscopy procedure. Method A total of 56 patients (53.5% females) who underwent colonoscopy procedure were included in this study. All patients underwent 12‐lead surface electrocardiograms (ECGs) just before colonoscopy and 15 minutes after colonoscopy. QT interval, QTc interval, Tp‐e interval, Tp‐e/QT, Tp‐e/QTc and frontal QRS‐T angle were calculated from 12‐lead ECGs. Results and discussion The frontal QRS‐T angle was significantly increased 15 minutes after colonoscopy compared to basal value (36.2 ± 24.3 vs. 29.5 ± 23.6, P = .003). In addition, repolarization parameters including QT, QTc and Tp‐e intervals were significantly prolonged at 15 minutes after colonoscopy compared to basal value, except Tp‐e/QT and Tp‐e/QTc. Significant ventricular or supraventricular arrhythmias were not observed in any patient during the procedure. What is new and conclusion In this study, we found that propofol administration increased the frontal QRS‐T angle in patients undergoing colonoscopy procedure. Given that a prolonged frontal QRS‐T angle is associated with ventricular arrhythmias, it may be safer to monitor those patients receiving propofol during colonoscopy procedures. Propofol is a most commonly used anesthetic drug for conscious sedation in colonoscopy procedures which may affect ventricular repolarization. There is no study investigating the effect of propofol on frontal QRS‐T angle. We have demonstrated that propofol administration increases the frontal QRS‐T angle in patients undergoing colonoscopy procedure. Therefore, we proposed that patients received propofol should be monitored closely in terms of ventricular arrhythmias.