In recent years, microwaves are considered a source of green materials in the manufacturing process. Thanks to its outstanding thermal and time capability, it was applied to many different areas of experimental and production scale. In this research, we optimized the yield of essential oils obtained from the Kaffir lime (Citrus hystrix) leaves in Vietnam by using the heat from the microwave oven extraction method. Researchers used heat from the microwave to extract polar compounds in plants because it is a green technique that improves the yield and quality of the essential oil extracted from the extraction process. Kaffir lime essential oil has many different effects in the food and cosmetic field because of its scent and antioxidant properties. The chemical composition of Kaffir lime leaves essential oil in this study was carried out by gas chromatography–mass spectrometry (GC-MS) method which showed that 19 constituents are accounting for 99.99% of the total essential oil with β-citronellal is the main constituent accounting for 81.23%, followed by β-citronellol (5.76%), citronellyl acetate (4.26%), linalool (1.85%), caryophyllene (1.51%) and other compounds accounted for less than 1.0% at 100 grams of Kaffir lime leaves. These compounds are known as effective antibacterial and antioxidant agents. In the ratio of raw materials to water (1:3 mL/g), extraction period (60 min), and microwave power (450W), optimum extraction conditions for the extraction of essential oil of Vietnam's kaffir lime (Citrus hystrix) leaves were achieved.