The effects of air-side partial premixing of dimethyl ether on laminar n-heptane/air counterflow flames were investigated through numerical simulation. Flame structures were computed with the profiles of flame temperature, mixture velocity, reactive species, and heat release rates, by solving the counterflow flame model with a comprehensive kinetic scheme of n-heptane and dimethyl ether. Two flame reaction zones were aroused in the flames with the addition of air-side premixing. The fuel-side nonpremixed reaction zone near the stagnation plane and the premixed one close to the air-side burner could be discerned by the distribution profiles and the production/consumption rates of some reactive species, as well as the heat release rates. The structural transition of the flames to the premixing equivalence ratio, phi(A) of the air-side mixture was characterized by the reaction zone thickness and the relative heat release intensity. The phi(A) values of 0.3 and 0.7 can be viewed as two changeover equivalence ratios. The variations of the flamelet temperatures against the premixing equivalence ratio were studied. Results showed that the thermal interactions between the two reaction zones are of importance in stabilizing the ultralean premixed flame for the air-side mixture and developing dilution combustion for the nonpremixed fuel flow.