This study was aimed to identify the objective understanding (OU) and subjective understanding (SU) of the informed consent and decisional regret (DR) and to determine the related factors to them in the cardiology clinical trials. The participants were 60 patients participating in the phase III clinical trials. Data was collected after giving informed consent form for OU and SU of the informed consent and 4 weeks after participating in the clinical trials for DR using self-reported questionnaires. Mean of OU, SU, and DR were 74.6, 58.6 and 42.9 based on 100 points, respectively. The predictors of OU were reading informed consent repeatedly (p=0.004), higher education than middle school (p<0.001), and time to giving information to consenting (p=0.034), and these variables explained 45.3% of OU variation. The predictors of SU were giving information by research nurses (p=0.025), higher education than middle school (p<0.001), making question before consenting (p=0.035), and these variables explained 48.8% of SU variation. The predictors of DR were giving information by research nurses (p=0.004), age (p=0.046), and these variables explained 23.6% of DR variation. SU showed negative correlation with DR (r=-0.34, p=0.008). In conclusion, the understanding of the informed consent, especially SU, was significantly lower and negatively correlated with DR. Therefore, it is needed to improve understanding of informed consent among participants in cardiology clinical trials to reduce DR.