<p>  Chills tend to generate easily in thin wall spherical graphite cast iron. Because chill is a hard and brittle structure, chill structures contained in cast iron decrease the mechanical properties of cast iron considerably, especially toughness. However, it is difficult to detect the existence of chills in cast iron by visual observation, and sometimes they may remain undetected in good cast iron products shipped out. In the present study, a non-destructive inspection method for detecting the existence of chills in cast iron instantaneously was reviewed by investigating the relationship between ultrasonic wave propagation factors (e. g. : ultrasonic velocity, propagation time, thickness of casting, etc.) experimentally. It was found that chills in the cast iron can be detected using virtual thickness S4 which is calculated from the 4th bottom echo of the ultrasonic pulse-echo method with constant ultrasonic speed and casting thickness.</p>