This research experimentally investigates the flame driving function in an acoustically excited vertical duct with a laminar, premixed flame anchored on a grid. The flame driving function is determined from the measured acoustic intensity upstream and downstream of the flame zone, which relates both the acoustic velocities and pressures on either side of the flame zone. Results of this research reveal that the location of the flame zone on the acoustic field plays the main role in determining the sign and magnitude of the flame driving function. The flame equivalence ratio plays a role only in the magnitude of the flame driving function. This study also shows that the characteristics of the flame driving function can be qualitatively predicted by knowing the imaginary part of nonreactive acoustic admittance at the flame location.