Happiness is a positive internal experience and an indicator of mental health. Having happy and efficient employees is one of the ideals of healthcare organizations, given its impact on the quality of healthcare services. Emergency healthcare staff members face various unpleasant and stressful events. It has been suggested that fragrant herbs such as Citrus aurantium can have cheerfulness effects on individuals. Therefore, this study was conducted to investigate the effect of Citrus aurantium aroma on the happiness of pre-hospital emergency staff. A randomized placebo controlled clinical trial was carried out on 167 prehospital emergency medical staff who were randomly assigned into two groups of aromatherapy and placebo. The participants received Citrus aurantium aromatherapy and placebo for five work shifts and for two consecutive weeks. Aromatherapy was performed using 10% Citrus aurantium scent applied via a pendant containing 1.5 cc of Citrus aurantium scent. The distance from the nose to the container was about 20 cm with the neck straight and the head in a balanced position. Data regarding staff happiness were collected using the Oxford Happiness Questionnaire, which were analyzed using descriptive and inferential statistics. The mean level of happiness did not differ between the intervention and control groups ( p = 0.99). Work experience had a statistically significant inverse association with happiness (coefficient = -0.76, 95% CI: -1.49, -0.03). The findings of this randomized controlled trial indicate that Citrus aurantium aroma has no effect on the happiness of prehospital emergency medical services workers. There is a need to study the effect of a combination of complementary and alternative methods on the creation of positive psychological feelings among pre-hospital emergency staff.