The pyrolysis mechanism of fuel under supercritical conditions is an important concern for developing regenerative cooling technology of advanced aircraft using hydrocarbon fuel as the primary coolant. n-Decane as a component of some jet fuels was studied at the temperature range from 773 to 943 K in a flow reactor under the pressure of 3, 4, and 5 MPa. Gas chromatograph/mass spectrometry was used to analyze the pyrolysis products, which were mainly alkanes from C-1-C-9 and alkenes from C-2-C-9. A kinetic model containing 164 species and 842 reactions has been developed and validated by the experimental results including the distributions of products and the chemical heat sink of fuel. The decomposition pathways of n-decane were illustrated through the reaction flux analysis. It is concluded that the C-4-C-9 alkanes are mainly generated by the recombinations of alkyls, while the small alkanes (C-1-C-3) are formed by H-abstraction reactions by C-1-C-3 alkyl radicals. The applicability at supercritical pressure and high fuel concentration condition of previous models was discussed, and the performance of the present model in reproducing the experimental data is reasonably good.