Problem caused by low Reynold’s number is one of main factors affect performance of high altitude endurance unmaned vehicle engines. Effects of low Reynold’s number on a small high bypass turbofan engine were discussed.The effects can make the performance of the engine’s components change when Reynold’s number decrease to some extend. Inlet blade chord Reynold’s Number of booster and low pressure turbine is lower than that of other components of the engine, and will be affected strongly with flight altitude increase and flight velocity decrease. The reduction of engine’s performance is due to component performance decline and engine re-match resulting from low Reynold’s effects on component performance. Not only efficiency of component, but also reduction of component’s mass flow, lead to the performance degradation of the engine. The rotator speed reduction (because of turbine inlet total temperature’s limit) enhances the low Reynold’s effects and hence reducing engine’s performance, engine thrust decrease rapidly, but increase of engine bypass ratio partly compensate the rise of specific fuel consumption caused by the component performance decline.