Abstract: A real time adaptive model for turbofan engine is developed,which uses Kalman Filter concept to estimate unmeasured output variables and characterizes the effects of off-nominal engine behavior as biases on the measurable output variables.An adaptive engine model is formulated and its block diagram is set up with this method.The estimates of the output variables obtained from the presented model are compared with nonlinear simulation results of the engine for testifying its adaptability to the off-nominal behavior of the engine with deteriorated performance.