The application of mild hydro cracking using former hydrodesulfurization commercial units to produce low sulfur high cetane number Diesel is a growing area of interest. The total pressure and the type of catalyst used are some of constrains associated with the quality of the product obtained from mild conversion. This paper focuses on the effect of fluid dynamics and vaporization on the product quality. Two sets of experiments have been carried out in a pilot plant to obtain information on the product and to adjust the kinetics constants. In addition, a hot separation system has been used to obtain samples of gas and liquid phase at reaction conditions. In this way the gas and liquid properties in the reactor were evaluated. The mass and energy balance for the trickle bed reactor were performed by using a simplified lump of kinetics. Additionally a commercial simulation package was used to calculate the vaporization. Information from cold model was used to carry out old up measurements. The information was important in order to interpret the vaporization and the type of molecules present in gas and in liquid phases. Using the kinetics model it was shown that the increase in vaporization reduce the gas phase residence time, hydrogen partial pressure, and hydrogen hydrocarbons ratio. By modifying the mild hydrocracking reaction temperature, the cetane number of the diesel fraction produced initially increases and then decrease when temperature is risen. Aromatic and naphthenes increase in the lighter fractions at high conversion.