This paper describes a computational fluid dynamics (CFD) methodology to simulate reactive flow within a cryogenic rocket nozzle and to predict the specific impulse(Isp). In this, the reactive flow is simulated by adding finite rate chemistry to standard compressible Navier-Stokes equations with proper turbulence model, thermodynamic model and reaction kinetics. Present work uses a six species, eight step reaction chemistry by JS Evans and CJ Schexnayder Jr for modelling Hydrogen-Oxygen reaction within the nozzle. This reactive flow simulation calculates the specific impulse of the nozzle after accounting for the boundary layer loss, kinetic loss, and divergence loss. Energy release loss is accounted by multiplying the predicted Isp value with C* efficiency obtained from experiment. The nozzle specific impulse predicted using this methodology was compared against test data reported during an experimental program conducted at altitude test capsule of NASA Lewis Rocket Engine Test Facility (RETF), where a contoured nozzle of area ratio (AR) 1030 and 427.5 were test fired with hydrogen-oxygen propellants at various mixture ratios. The present reacting flow solver was able to successfully simulate nozzle flow field and could predict delivered specific impulse within an accuracy range of 0.1-1.82% . CFD results were compared with Isp values obtained from Joint Army, Navy, NASA, Air Force (JANNAF) performance prediction procedure. The validated method was used for predicting the Isp value for 100AR cryogenic nozzle which powers the final stage of ISRO's launch vehicle. Copyright © 2018 by the International Astronautical Federation (IAF). All rights reserved.