ABSTRACT The main goal of this research work is to theoretically investigate the superconductivity of Nano size Indium (In). The correlation between the electron phonon coupling parameter (), the Coulomb’s pseudo potential (*) and the transition temperature at reduced scale is determined by using the McMillan’s strong-coupling model. Using the strong coupling theory, the size dependence of the transition temperature of superconducting Nano size Indium (In) is derived. The expression for transition temperature has been determined and plotted as a function of the electron phonon coupling parameter and the coulomb’s pseudo-potential for superconducting Nano size Indium. The dependence of the critical temperature (T C ) on the electron-phonon coupling parameter  is demonstrated by plotting a phase diagram. Keywords: Superconductivity, McMillan equation, Transition temperature, Nano size Indium.