The behavior of a diffusion flame under acoustic excitation was experimentally studied with the aid of simultaneous flow visualizations of the flame combining Mie scattering and schlieren/shadowgraph imaging. The Mie scattering imaging showed the periodic formation of a vortex street along the flame axis, while the schlieren/shadowgraph images indicated the formation of a high-amplitude density fluctuation near the burner exit. Furthermore, mean velocity and temperature along the flame axis were measured for evaluating the flame behavior by acoustic excitation. These experimental results showed that the acoustic excitation led to a density fluctuation near the burner and promoted the flame behavior near the burner. This behavior resulted in enhanced mixing and combustion in the flame near the burner, which grew as the oscillation amplitude of the acoustic excitation increased. In addition, the flame height, oscillation amplitude and luminosity decreased as the acoustic excitation amplitude is highly increased because of the formation of the increased velocity region with high-temperature near the burner associated with the initiation of laminar-to-turbulent transition of the flame.