Background Ethical decision‑making and behavior of nurses are major factors that can affect the quality of nursing care. Moral development of nurses to making better ethical decision-making is an essential element for managing the care process. The main aim of this study was to examine and comparison the effect of training in ethical decision-making through lectures and group discussions on nurses’ moral reasoning, moral distress and moral sensitivity. Methods In this randomized clinical trial study with a pre- and post-test design, (...) 66 nurses with moral reasoning scores lower than the average of the community were randomly assigned into three equal groups (n = 22) including two experimental groups and one control group. Ethical decision-making training to experimental groups was provided through the lectures and group discussions. While, the control group did not receive any training. Data were collected using sociodemographic questionnaire, the nursing dilemma test (NDT), the moral distress scale (MDS) and the moral sensitivity questionnaire (MSQ). Unadjusted and adjusted binary logistic regression analysis was reported using the odds ratio (OR) and 95% confidence intervals. Results Adjusted regression analysis showed that the probability of increasing the nursing principle thinking (NPT) score through discussion training was significantly higher than lecture (OR: 13.078, 95% CI: 3.238–15.954, P = 0.008), as well as lecture (OR: 14.329, 95% CI: 16.171–2.005, P < 0.001) and discussion groups compared to the control group (OR: 18.01, 95% CI: 22.15–5.834, P < 0.001). The possibility of increasing moral sensitivity score through discussion training was significantly higher than lecture (OR: 10.874, 95%CI: 6.043–12.886, P = 0.005) and control group (OR: 13.077, 95%CI: 8.454–16.774, P = 0.002). Moreover, the moral distress score was significantly reduced only in the trained group compared to the control, and no significant difference was observed between the experimental groups; lecture group vs. control group (OR: 0.105, 95% CI: 0.015–0.717, P = 0.021) and discussion group vs. control group (OR: 0.089, 95% CI: 0.015–0.547, P = 0.009). Conclusions The results of this study indicate that ethical decision-making training is effective on empowerment of ethical reasoning. Whereas the group discussion was also effective on increasing the ethical sensitivity, it is recommended the training plan provided in this study to be held as workshop for all nurses in health and treatment centers and placed in curricular plan of nursing students. Registration This randomized clinical trial was registered in Iranian Registry of Clinical Trials under code (IRCT2015122116163N5) in 02/07/2016. (shrink)