In order to elucidate the blowoff mechanism of burner flames, mass burning velocities, tangential components of mass velocities to the ignition front and stretching near the flame base were examined. Experimental results show that the mass burning velocity of a stabilizing point in the flame base at the blowoff limit is larger than that at the lean limit of inflammability. Therefore, it is not a valid interpretation that blowoff occurs as the fuel concentration of the stabilizing point approaches that of the lean limit of inflammability, because of dilution with the surrounding air. Since stretching of the flame at the stabilizing point has a tendency to decrease when approaching the blowoff, the flame-stretch concept can not be applied to it. A model for the stabilization mechanism of premixed flames, in which the boundary velocity gradient theory is expressed algebraically, is proposed to explain the blowoff the burner flame, as well as the inverted flame reported previously by the authors.