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In order to elucidate evaporation process of organic molecular beam deposition (OMBD) technique, the molecular flux and its dependence on the Knudsen-cell temperature, are directly measured by using a high performance quadrupole mass spectrometer in an ultra high vacuum. Functional organic materials such as metal phthalocyanine (M = Cu, Ni, Pb and TiO), Tris(8-hydroxyquinoline) aluminum (Alq3), Triphenyl diamine (TPD) and fullerene (C60), are sublimed from the K-cell which was precisely controlled the temperature. The mass spectrum revealed that each molecule was sublimed from K-cell without any thermal decomposition. The dependence of evaporation rate on the temperature of K-cell has been, for the first time, numerically calibrated as the thermodynamical enthalpy for sublimation as a basic and important factor.