The inherent compressibility of biodiesel has strong impact on its dynamic properties, particularly under high temperature and pressure conditions. In order to clarify the connection between the microscopic chemical structure of organic liquids and their macroscopic compressibility, a qualitative method is proposed by using Molecular Adiabatic Compressibility (MAC). By analyzing sound velocity and density data of methyl and ethyl caprate, two-dimensional fitting expressions of MAC obtained in isobaric and isothermal curves respectively. Partial derivatives of two-dimensional expressions for pressure and temperature are introduced to quantify the dependency of MAC upon these parameters. Further, a sets of new three-dimensional expressions for MAC are proposed to demonstrate these dependency characteristics in P-T domain under the fuel working conditions of the injection system. Based on the definition of MAC, a new state equation was developed by integration of 3D fitting expression. The calculation results of molar volume indicate that the integral state equation is equivalent to the ESD model. The new fitting expression successfully captures the influence of temperature and pressure on the compressibility of organic liquid fuels, and reveal higher calculation accuracy of molar volume under extreme high pressure. (C) 2015 Elsevier Ltd. All rights reserved.