The composition of essential oils extracted from plants can present great variability, even between individuals of the same species due to different conditions, such as extraction methods, climate and soil composition, among others. Most methods available for the determination of essential oil composition, such as gas chromatography-mass spectrometry (GC/MS), are expensive and time consuming, which prevents farmers or producers from determining the true value of their product or performing in-line quality control in production. Therefore, practical tools are required to test the quality of essential oils in a quick and inexpensive way. The aim of this study was to determine the relationship between the concentration of thymol and refractive index of essential oils as a quality control measure in Lippia origanoides Kunth, an aromatic and medicinal plant with a large content of thymol. Essential oils were extracted by steam distillation from leaves of 5-month-old plants. A Clevenger type apparatus was used for extraction. The refractive index was measured with an Abbe refractometer, while the concentration of thymol was determined by GC-MS. It was found that thymol concentration and refractive index are adjusted in a linear equation y = 0.00030x + 1.4893 with a correlation coefficient of 0.9439, demonstrating that the refractive index of the essential oils of Lippia origanoides Kunth can be used to determine the thymol concentration of the oil.