There are two methods to demonstrate the critical flow rate of one-dimensional steady isentropic flow when Mach number is one at the throat of nozzle.However,the first one is restricted to the ideal gases with constant specific heat,and the second is based on the incorrect equations.A simple,concise,and rigid method based on the isentropic and adiabatic energy equations to prove the condition of critical flowrate for one dimensional isentropic flow is given in the paper.