Bead-on-plate welding is carried out with an IPG YLS-6000 fiber laser and a slab DC035 CO2 laser under the similar conditions. The melting efficiencies are calculated based on the measured weld cross-sectional areas with an Olympus microscope. The results show that the melting efficiency increases firstly and then decreases with welding speed increasing in both cases. However, the welding speed corresponding to the maximum melting efficiency is far greater in fiber laser welding than that in CO2 laser welding. Further analysis demonstrates that the difference of the melting efficiency is relevant to the inherent difference of the energy coupling. Therefore, from the viewpoint of welding efficiency, fiber laser is more suitable for high-speed welding, while CO2 laser is more suitable for low-speed welding.