Abstract: By using the WKB method and slowly-varying wave packet theory the main cause for the size change in Tropical Cyclone is analyzed. It is shown that the size change in Tropical Cyclone is mainly determined by the inner factors. In general case the angular frequencies of inertio-gravity waves are positive, thus provided that the both stratification and inertia are stable, when the parameters of inertial and static stabilities increase with the increasing distance from centre the size is increased; when the parameters of inertial and static stabilitics decrease with the increasing distance from centre the size is decreased. The former may be called the “expanding” of Tropical Cyclone and the latter may be called the “shrinking” of Tropical Cyclone.