The research is about the determination of developing flow pattern in the axial flow fan impeller. A measuring method and a sizing method of axial flow fans used in a research project have been studied. In this way it has been determined the spatial distribution of static pressure directly behind the impeller as well as the spatial distribution and direction of total pressure in a fixed system to the blade channel. There is an opportunity to identify the high loss region in the impeller: to determine the pressure loss and the hydraulic efficiency at each point. The article shows the results of comparison between the measurement data and the sizing results in the case of one in the agricultural-field utilized cooling fan. The impeller was designed by the help of the fore slanted blades in circumference direction as well as the non-free vortex theory. The basic theories of the design method of the blades and the realization of measurement results are analyzed, too.