A 55-species reduced mechanism for n-heptane oxidation was derived from a 188-species skeletal mechanism developed previously from the detailed 561-species mechanism of LLNL. This 188-species skeletal mechanism was first reduced by using directed relation graph, together with sensitivity analysis, resulting in a 78-species skeletal mechanism. An isomer lumping approach was then implemented to group isomers with similar thermal and diffusion properties into composite species, and reactions involving the grouped isomers were lumped accordingly. It was found that the intra-group fraction of the isomer concentrations in the composite species remains almost constant in a wide range of parameters such that the lumped reactions remain in elementary form. A 68-species mechanism was thus obtained by eliminating 10 species through isomer lumping. Forty-two unimportant reactions were further eliminated by comparing their contributions to the production rate of each species. Finally, 13 global quasi steady state (QSS) species were identified through time-scale analysis, resulting in the 55-species reduced mechanism with 283 elementary reactions lumped to 51 semi-global steps. The concentrations of the QSS species were solved analytically with high efficiency. Validation of the reduced mechanism shows good agreement with the detailed mechanism for both ignition and extinction phenomena.