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Laminar burning velocities and Markstein lengths of premixed methane−n-heptane−air mixtures were experimentally investigated at an initial pressure of 0.1 MPa, initial temperatures of 358, 393, and 428 K, and equivalence ratios of 0.7−1.5. The methane content in the methane−n-heptane mixtures ranges from 0 to 1. The experiments were conducted in a combustion chamber with central ignition. In the present study, the difference between the linear and nonlinear extrapolation methods was analyzed quantitatively for methane−n-heptane−air flames. Comparisons of the laminar burning velocities of methane−air and n-heptane−air flames were conducted, respectively, between the present and other studies. Subsequently, effects of the initial temperature and methane content on the laminar burning velocity and flame instability of methane−n-heptane−air mixtures were analyzed. Then, the critical methane content at which the laminar burning velocity and flame instability of methane−n-heptane−air flames start to change relatively significantly was explored. The results show that the laminar burning velocities of methane−air and n-heptane−air flames measured in the present study are in good agreement with the data available in the literature. Laminar burning velocities and flame instabilities of methane−n-heptane−air flames seem to be less sensitive to the methane content when the methane content is below 0.75. The change in the initial temperature tends to diminish the difference in the laminar burning velocity between methane−air and n-heptane−air flames and only has weak effects on the flame instability of methane−n-heptane−air mixtures. According to variations of the laminar burning velocities and Markstein lengths of methane−n-heptane−air flames with the methane content, the value of 0.75 can be treated preliminarily as the critical methane content.