In Stranski-Krastanov mode, the transition from two-dimensional growth to three-dimensional growth occurs at a certain thickness: a transition thickness. In subsequent growth, misfit dislocations are incorporated at the inter-face between the substrate and the epitaxially grown layer. This limited thickness for coherent growth is called: the critical thickness. An analytical equation for the transition thickness is derived by the thermodynamical approach in which strain energy and surface energy are taken into account. The three-dimensional island, often called a quantum dot, is stably formed, when the strain energy of the two-dimensional layer exceeds the sum of the relaxed strain energy and the excess surface energy of the three-dimensional island. The equation is applied to obtain the transition thickness of InAs/GaAs, InP/ GaP, and Ge/Si which fit fairly well with the experimental results.