A pressure type homogenizer consists of a plunger pump, and a valve part with a narrow channel. It generates effects such as a shear, a turbulent flow and a cavitation between fluid molecules at very short time, and makes a homogeneous emulsification state. If the flow pattern in a valve can be solved in more detail, the more efficient and stabilized emulsification will be attained. However, since it is difficult to measure the size, the pressure and the flow velocity in a narrow gap space by experiment, the details of the mechanism of emulsification by s shear, a turbulent flow, a cavitation and a collision have not been clarified enough. In this research, the size of gap is first calculated from the theoretical equation assumed to be laminar flow, based on the Reynolds number of the gap flow. Next, the pressure, the flow velocity, shearing stress, etc. are calculated. Then, the emulsification mechanism of each part of the valve is clarified by expressing the droplet size as a function of the gap size, the shearing stress and the exit velocity about two kinds of original solutions in which interfacial tension differs. Furthermore, the prediction expression of droplet size obtained by the emulsification is proposed.