Background & Aim: Melissa officinalis (lemon balm), belongs to the family Lamiaceae is a known herb that has long been used in traditional medicine to treat many disorders and give fragrance to different food and beverage products. In this study, the effects of the foliar spray of L-phenylalanine on essential oil content and chemical composition of lemon balm (Melissa officinalis), at field condition in Horeh, Saman city of Chaharmahal and Bakhtiari Province, southwestern Iran, were investigated. Experimental: This study was conducted in randomized complete block design with three replications. Experimental treatments were including three levels of the foliar application of L-phenylalanine including 1000, 2000 and 4000 mg/L. Distilled water was used as a control. Results: The essential oil contents of the treatments ranged between 0.11 to 0.19% (v/w). Statistical analysis indicated that there were significant differences among treatments for essential oil contents of M. officinalis. Neral, Z-Citral, E-Citral, Geranial, Citronellal and β-Caryophyllene were the six major compounds of M. officinalis essential oil. The results indicated that the foliar application of L-phenylalanine have significant impacts on main constitutes of the essential oil and generally the application of L-phenylalanine spraying have beneficial and effective role in improving the essential oil content and some of chemical composition of lemon balm. Recommended applications/industries: The plant biostimulants as environmentally friendly products can be effective for increasing the quantity and quality of lemon balm essential oil. Application of L-phenylalanine can be a promising strategy in achieving organic production of medicinal plants such as lemon balm. Manuscript profile