A new second order group contribution method for the estimation of critical constants of organic compounds is developed. The functional groups of Joback and Reid method are employed with some changes as first order functional groups and some second order functional groups are also defined and added to them. A set of experimental data for the critical temperature of 969 compounds, the critical pressure of 715 compounds and the critical volume of 539 compounds are collected in this study. The group contribution values of 122 functional groups have been optimized using the experimental data of organic compounds, including complex and heavy polycyclic aromatic components. The critical constants of most structural isomers and stereoisomers are distinguished in this method by considering the second order functional groups. The hydrocarbons and hydrocarbon derivatives considered in this research include carbon, hydrogen, oxygen, nitrogen, sulfur, fluorine, chlorine, bromine, and iodine atoms. Development of the new models and optimizing the values for the group contributions have been performed by the fminsearch algorithm in MATLAB software. The results of the new method are compared to three well known group contribution methods. The percentage of the average absolute relative deviations are 1.12% for critical temperature, 2.69% for critical pressure, and 2.67% for critical volume. (C) 2019 Elsevier B.V. All rights reserved.