The possibility negative specific heat was first suggested in 1968 and subsequently explained by Thirring, Beckenstein and Hawking. The research related to negative specific heat continues till recent time in various topics such as self-gravitating N-body system, thermodynamics of black holes, large-N thermal QCD, Fermionic King model in self gravitating system, finite non-interacting Bose gases and systems with wave-particle interaction. It is also discussed in the context of out-of-equilibrium canonical ensemble. We noticed 'S'-shape in the temperature dependence of energy and resulting negative specific heat in finite isolated system around 2000 in 2D Potts model. Such 'S'-bend was observed in the phase transition region of 2D system consisting of 870 hard-disk particles in 1962. Some of these work are theoretical possibilities and some suggests experimental verification. We have concluded using extremely large Monte Carlo step per spin that such 'S'-shape to be the equilibrium response of a finite isolated system undergoing 1st order transition. We applied it to XY model and Potts model.