Moisture in gas turbine inlet air can affect its operation and performance. Therefore, this study investigates the effect of humidity on the steady-state and transient performance of a single-spool micro turbojet engine at different conditions. The thermodynamic model of the engine was created using the inter-component volume (ICV) method and run in the MATLAB/Simulink environment. The effects of humidity are modelled by considering the effect of water vapor on gas properties. We also examined performance degradation due to the impact of compressor fouling on engine parameters, and the effect of humidity on compressor fouling. The results obtained from the Simulink model have been validated with the results of the GasTurb software.. The results indicate that at the highest humidity levels without compressor fouling, the change in parameters was modest, with a maximum deviation of approximately 1%. If fouling is present at zero relative humidity, the thrust decreases by 4 percent and the specific fuel consumption increases by 5 percent. At 100% relative humidity with fouling, the thrust decreases by 8 percent and the specific fuel consumption increases by 10 percent. In the transient process, the speed of response decreases if fouling occurs, and this response becomes slower with increasing relative humidity.