We need to know the thermo-physical properties of different compounds in order to design, model or optimize the industrial processes. Among various thermo-physical properties, enthalpy of vaporization at normal boiling temperature is one of the most important and useful properties in industrial processes. Although experimental data are available from various sources, but in order to complete these data, theoretical methods may be required to estimate and predict this property. The new group contribution method which can be regarded as a modification to the original Joback and Reid method gives more accurate estimations of the enthalpy of vaporization at normal boiling temperature of organic compounds. Group contribution values have been optimized using 3950 experimental data of organic components with a molecular weight range of 28-565 (g mol(-1)) and number of carbon atoms range of 1-40. Checking the results of the new method on 3950 different organic compounds shows an average absolute error of 0.620 kJ/mol and the percentage average relative error of 1.683%. The results of the new method have been compared with the original method of Joback and Reid, and the Kolska, Ruzicka and Gani method. The results showed the better agreement between the predictions of the present method and the experimental data, comparing to the other group contribution methods. (C) 2018 Elsevier B.V. All rights reserved.