Abstract: Abstract A method,“gross thrust model”,is developed to determine gross thrust of jet engine and afterburning turbofan engine in flight testing.This model is able to determine the gross thrust of modern engines with an acceptable degree of accuracy only by using a few of pressure measurements.The gross thrust model is based on a one dimensional analysis of flows in mixer,afterburner and nozzle.Some correction coefficients are required to account for three dimensional effects,the effects of friction and mass transfer,as well as the effects of the simplifying assumptions used in the theory.The correction coefficients in the model are K,C and E.Both K and C are constants,whereas E varies with engine operating conditions.In order to determine the reasonable coefficients K,C and E,it is necessary to acquire engine testing data in the altitude facility at flight conditions as much as possible.A comparison was made between the data determination from the gross thrust model and the gross thrust measured from the altitude facility for engine J85 GE 21 at H=7620m, Ma 0=0 90.It demonstrates that the accuracy of the gross thrust model is about 3 0 percent and depends on the pressure difference between the total pressure of the turbine discharge and the static pressure of the nozzle inlet.