Abstarct: In this work, the linear isotherm regularity, LIR, was extended to dense nonlinear organic fluids using the group contribution method. For this purpose, by introducing two new groups namely, tertiary carbon atom ( ) and quaternary carbon atom ( ) for nonlinear organic compounds, each of these compounds have been assumed as a hypothetical mixture of methyl and methylene groups, tertiary and quaternary carbon atoms and the other constituent functional groups. Then, the LIR equation of state (EOS) has been extended to such a hypothetical mixture. The accuracy of the modified LIR equation of state (MLIR) was investigated by plotting of (Z/n-1)v2 versus 2 for different isotherms of nonlinear organic compounds using experimental pVT data and their linearity were confirmed (R2 ≥ 0.9974). In the next step, to obtain the contribution of these two new groups; tertiary carbon atom and quaternary carbon atom, in the parameters of the MLIR equation of state, two new basic compounds were introduced: 2-methylpentane for the contribution of tertiary carbon atom ( ) and 2,2,4-trimethylpentane for the contribution of quaternary carbon atom ( ). Using the values of MLIR equation of state parameters for the previous and 2 newly basic compounds, the contribution of the constituent groups for 16 different nonlinear dense organic fluids in the parameters of MLIR equation of state at any given temperature was calculated. Then, by calculating the value of MLIR equation of state parameters for these compounds along with the EoS, their density was predicted in terms of pressure at the given temperatures.