Abstract : Black holes are classically black but in quantum theory they behave like a thermodynamic object in particular it has entropy. Also the study of black holes have provided many insights about, yet to be known, quantum theory of gravity. One of the insight is the principle of holography and in string theory which is a candidate of quantum theory of gravity, we have a mathematical realization of this holography in the form of AdS/CFT correspondence. Based on AdS/CFT there is a proposal to compute the full quantum entropy of an extremal black hole. In this talk I will discuss some thermodynamic properties of a black hole and computation of quantum correction to the entropy of extremal and non extremal black hole.