A nanoscale calorimeter using nanohole arrays for measuring the latent heat of transformation associated with the thermodynamically first-order antiferromagnetic-ferromagnetic phase transition in FeRh thin film is described. The nanocalorimeter design is based on a surface plasmon phenomenon that is observed using a nanohole array sensor composed of holes that are 150 nm in diameter and spaced 350 nm apart that are excited by a helium-neon laser. The extraordinary optical transmission (EOT) measured on the backside of the film is used to determine the thermal history of the transition; this information will be employed to quantitatively determine the energy release associated with this magnetic transition. A numerical technique based on the Stefan problem employing thermodynamic properties for evaluating enthalpy changes in phase transitions is described and illustrates the proposed calorimetric process. The method analyzes a first-order phase change in a magnetic material with density ?, Curie temperature Tc, constant specific heat Cp, and thermal conductivity k. One aim is to demonstrate that the numerical method successfully predicts the temperature histories for the magnetic phase change. When the temperature histories are experimentally obtained, an inverse method is applied to calculate the enthalpy released. The numerical results will be compared with experimental results. It is anticipated that, this nanoscale calorimeter will have potential applications for the characterization of FeRh thin films and other functional thin films.