Computational Fluid dynamics (CFD) analysis was used to evaluate the heat transfer performance during the freezing process in cryotherapy. Cryoprobes are generally used for the treatment of diseased tissue in cryotherapy. As the hydraulic diameter of the cryoprobe is very small, it was considered as a mini/microchannel. CFD software packages FLUENT and GAMBIT were used for the simulation of a cryoprobe. Nitrous oxide at a very temperature of 183K was used as the working fluid. The simulation was carried out for different channels with hydraulic diameter varying from 20μm to 1500μm. Reynolds number were varied between 0.01 and 10 in order to vary the velocity at the inlet. It was found that with an increase in hydraulic diameter, there was a delay in the saturation boiling of the working fluid. The investigation also shows that the delay in boiling process occurs with an increase in Reynolds number as well. The pressure drop decreases with an increase in the hydraulic diameter of the channel. On the other hand, pressure drop increased with an increase in the Reynolds number.