This study evaluated the effects of inhalation of Lavender essential oil on vital signs in open heart surgery ICU. The main complaint of patients after open-heart surgery is dysrhythmia, tachycardia, and hypertension due to stress and pain. Due to the side effects of chemical drugs, such as opioids, use of non-invasive methods such as aromatherapy for relieving stress and pain parallel to chemical agents could be an important way to decrease the dose and side effects of analgesics. In a multicenter, single-blind trial, 40 patients who had open-heart surgery were recruited. Inclusion criteria were full consciousness, lack of hemorrhage, heart rate >60 beats/min, systolic blood pressure >100 mmHg, and diastolic blood pressure >60 mmHg, not using beta blockers in the operating room or ICU, no history of addiction to opioids or use of analgesics in regular, spontaneous breathing ability and not receiving synthetic opioids within 2 h before extubation. Ten minutes after extubation, the patients >vital signs [including BP, HR, Central Venous Pressure (CVP), SPO2, and RR] were measured. Then, a cotton swab, which was impregnated with 2 drops of Lavender essential oil 2%, was placed in patients' oxygen mask and patients breathed for 10 min. Thirty minutes after aromatherapy, the vital signs were measured again. Main objective of this study was the change in vital sign before and after aromatherapy. Statistical significance was accepted for P < 0.05. There was a significant difference in systolic blood pressure (p 0.001 <), diastolic blood pressure (p = 0.001), and heart rate (p = 0.03) before and after the intervention using paired t-test. Although, the results did not show any significant difference in respiratory rate (p = 0.1), SpO2 (p = 0.5) and CVP (p = 0.2) before and after inhaling Lavender essential oil. Therefore, the aromatherapy could effectively reduce blood pressure and heart rate in patients admitted to the open heart surgery ICU and can be used as an independent nursing intervention in stabilizing mentioned vital signs. The limitations of our study were sample size and lack of control group. Randomized clinical trials with larger sample size are recommended.