An accurate correlation to predict the thermal pressure coefficients of normal paraffins, including polyethylene, is obtained, Only the number of carbon atoms is needed for the prediction of thermal pressure coefficients at densities from triple point up to critical point. The relationship between the thermal pressure coefficients of branched paraffins and their molecular structures is also discussed based on the graph theory. Two topological indexes, the Wiener index and polarity number, were used to develop a correlation which can predict the thermal pressure coefficients for branched paraffins satisfactorily.