The atomizing characteristics in a spray injected from a twin fluid atomization nozzle have been investigated. The Sauter mean diameters as mean diameter are compared with wavelength calculated from the instability theory. The Sauter mean diameter are measured by the Fraunhofer diffraction theory using the Malvern particle sizer. The wavelength is calculated using the mean relative velocity instead of the exit relative velocity of nozzle. Also shadowgraphy technique is used to visualize atomization phenomena. This paper gives a possibility that the mean diameter can be predicted with the wavelength obtained by the simple instability theory.