Organic molecular devices create a need for establishing preparation methods of functional organic ultrathin films by the organic molecular beam deposition. It is particularly important that the molecular flux is precisely controlled in the ultrahigh vacuum. However, there are only a few studies on molecular flux of organic materials and in these a high performance quadrupole mass spectrometer was applied to the direct observation of the mass distribution and evaporation rate for various organic compounds, such as fullerene, metal phthalocyanines, and so on. In this paper, it is confirmed that the molecular flux from a Knudsen cell (K-cell) can be limited to single molecules and controlled under a constant evaporation rate. Also, the dependence of the deposition and evaporation rate on the temperature of K-cell was examined. As a result, the thermodynamical enthalpy of evaporation on these organic materials was determined for the first time and quantitatively estimated for the molecular flux.