The molecular compressibility, which is a macroscopic quantity to reveal the microcompressibility by additivity of molecular constitutions, is considered as a fixed value for specific organic liquids. In this study, we introduced two calculated expressions of molecular adiabatic compressibility to demonstrate its pressure and temperature dependency. The first one was developed from Wada’s constant expression based on experimental data of density and sound velocity. Secondly, by introducing the 2D fitting expressions and their partial derivative of pressure and temperature, molecular compressibility dependency was analyzed further, and a 3D fitting expression was obtained from the calculated data of the first one. The third was derived with introducing the pressure and temperature correction factors based on analogy to Lennard-Jones potential function and energy equipartition theorem. In wide range of temperatures and pressures , which represent the typical values used in dynamic injection process for diesel engines, the calculated results consistency of three formulas demonstrated their effectiveness with the maximum 0.5384% OARD; meanwhile, the dependency on pressure and temperature of molecular compressibility was certified. 1. Introduction In modern diesel engine, the injection pressure of fuel spray system continuously was elevated to achieve better atomization, combustion, and emission effect and even reach 200？MPa or more, which increase vastly the dynamics sensibility of fuel compressibility. The dynamic delivery process, which was carried out under rapid variation of pressure and temperature, was strongly affected by the compressibility of fuel and its derivative properties relative to pressure and temperature. For example, density influences the conversion of volume flow rate into mass flow rate [1], whereas the compressibility (the reciprocal of bulk modulus) acts on the fuel injection timing [2, 3]. Therefore, the adaptation of injection systems to biodiesels requires an accurate knowledge of the volumetric properties of biodiesel components over a wide range of pressure and temperature [4, 5]; especially the compressibility and sound velocity values were indispensable parameters for system modeling and experimental injection rate determination [6]. In order to clarify the connection between the microscopic chemical structure of organic liquids and the compressibility from investigation of sound velocity and density, many theoretical analyses and derivations have been performed to determine these properties. Rao [7] has pointed out the
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