A systematic approach was developed to derive non-stiff reduced mechanisms for direct numerical simulations (DNS) with explicit integration solvers. The stiffness reduction was achieved through on-the-fly elimination of short time scales induced by two features of fast chemical reactivity, namely quasi steady state (QSS) species and partial equilibrium (PE) reactions. The sparse algebraic equations resulting from QSS and PE approximations were utilized such that the efficiency of the on-the-fly stiffness reduction is high compared to the general methods of timescale reduction based on Jacobian analysis. Based on a previous 55-species reduced mechanism for n-heptane oxidation, developed from an LLNL detailed mechanism with 561 species, a non-stiff mechanism with 52-species mechanism was obtained. This mechanism was first validated for ignition and extinction applications over the parameter range of equivalence ratio 0.5∼1.5, pressure 10∼50atms, and initial temperature 700∼1600K for ignition, showing a worst case error of about 30%, and then applied in 0-D and 1-D unsteady flows with fixed integration time step of 10ns. It was demonstrated that the chemical stiffness was successfully removed, the integration was numerically stable, and the error induced by the stiffness removal procedure was small.