This paper deals with the application of 1-D simulation technique for prediction of engine performance at high altitudes. 1-D engine simulation is an important tool for engine development activities. Engine design through simulation can substantially cut down time needed to execute experiments and prototyping, as todays softwares can simulate most of the experiments. This approach was applied for simulation of a spark-ignited engine for UAV. A detailed 1-D thermodynamic model was prepared for the engine configuration in Ricardo WAVE environment; different simulation runs were executed and then performance parameters like brake power, torque, specific fuel consumption, BMEP, in-cylinder pressure etc. were predicted. In 1-D simulation technique, predicted performance shall always be co-related with test results for correct interpretation. Subsequently the simulated results of 1-D thermodynamic model were validated with test results in the present case; both cycle average parameters and instantaneous parameters were used to validate the thermodynamic model.A validated model of high accuracy level is advantageous and can be applied with greater confidence to optimize engine parameters. The validated model is an important and milestone step for any engine development process. In the present case the validated model was used to predict high altitude performance of UAV engine i.e. power at different altitudes. High altitude performance for UAV engine is always critical, as it is a deciding factor for UAVs in terms of climb rate, service ceiling, max. speed, etc. Further simulated high altitude performance of the engine was then compared with high altitude test results and showed good co-relation. The model validated at first place responded with ease with varying environmental conditions. To sum up, this paper flows through the approach used by us in defining the 1-D thermodynamic model, its simulation, methodology for model validation and lastly its application to predict high altitude performance.