Temperature dependence of the quenching efficiency of Mg(3s3p P-1(1)) by CH4 collision in the Ar bath gas has been studied over the temperature range of 660-850 K, using a pump-probe technique with time-resolved laser-induced fluorescence (LIF) as detection. The obtained thermal rate coefficients are attributed to physical and chemical quenching. The former contribution is evaluated to be less than 6.7 x 10(-11) cm(3) molecule(-1) s(-1), while the latter one is (0.59-1.12) x 10(-11) exp(5.75-6.54 kcal mol(-1)/RT) cm(3) molecule(-1) s(-1) (R is gas constant; T is temperature). The chemical reaction dominates over the exit channels, especially in the low temperature. The negative temperature dependence indicates the existence of a depth-well intermediate. The observed depth-well energy below the reactants is about twice as small as that evaluated theoretically. This bound state is anticipated to locate in the region of surface crossing between the excited and the ground states. We adopted several models to account for the kinetic data as a function of temperature. The orbiting and absorbing-sphere models cannot be validly applied to the current system. The angle-dependent line of normals (ADLN) model takes into account the angular dependence of the threshold energy. The ADLN fit to the measured kinetic data is consistent with the insertion mechanism reported previously, which shows energetic preference to the end-on attack.