Thermal cracking of a series of model compounds (n-octane, n-decane, n-dodecane, cyclohexane, methylcyclohexane), as well as a commercial Chinese jet fuel RP-3, was performed in a flowing reactor consisting of an electrically heated tube under a pressure of 5 MPa to obtain the chemical compositions of the cracked hydrocarbon fuels at different levels of cracking conversion. The phase envelopes and critical points of the cracked hydrocarbon fuels were calculated using the Peng-Robinson and Soave-Redlich-Kwong equations. The calculation results showed that the critical points for cracked hydrocarbon fuels are strongly dependent on the hydrocarbon type and cracking conversion and that the critical temperature of cracked fuel decreases but the critical pressure increases sharply from 2-4 to above 10 MPa because of the appearance of many small-molecule products. Therefore, phase changes of the hydrocarbon fuel from compressed liquid phase to supercritical phase and then to gas phase possibly occurred in the electrically heated tube reactor when the cracking conversions exceeded 30%. Based on the calculation results, the supercritical cracking of hydrocarbon fuels should be carefully used at higher cracking conversions of hydrocarbon fuels under high pressure.