THEORY OF TIME SCALE IN STRUCTURAL ISOMERIZATION OF CLUSTERS Tomohiro Yanao and Kazuo Takatsuka Graduate School of Arts and Sciences, The University of Tokyo, Tokyo, Japan A statistical theory which accounts for the dependence of average lifetimes of locally stable structures of Ar\raisebox{-0.7ex}{\scriptsize 7}-like clusters on the total internal energy is presented. This theory is based on a hypothesis about the geometrical structures of potential surfaces and is valid for clusters of high energy which are in the so-called liquidlike phase.