Stabilization and autoignition mechanisms of lifted flames have been widely investigated to improve combustion efficiency and safety of combustion equipment. This paper focuses on liftoff behavior and combustion characteristic of methane and propane flames under various coflow conditions in a coflow burner. Unlike the case of free jet flame in ambient air, the different tendencies of liftoff height changes with jet velocity for both methane and propane flames in vitiated coflow illustrate a transition from conventional combustion to Moderate & Intense Low Oxygen Dilution (MILD) combustion. Flame temperature difference with radial position measured by primary spectrum pyrometry proves the transition regime.