The energy coupling rate during welding of Al-Mg-Si alloys with a CO₂ laser and a Nd:YAG laser was measured calorimetrically. The coupling rate obtained with the Nd:YAG laser was higher than that with the CO₂ laser for both heat conduction welding and keyhole welding. The coupling rate for CO₂ laser welding changed steeply at the transition from a heat conduction welding state to a keyhole welding state. Coupling rates under the keyhole welding state match the Gouffe-Dausinger model that determines the coupling rate based on the weld depth and focal spot size. This model includes a constant figure that is dependent on laser beam absorptivity to molten metals and plasma in the keyhole. It is thought that the coupling rate in the heat conduction state depends on the absorptivity of the laser beam to molten metals. Absorptivity to plasma in the keyhole also affects the coupling rate in the keyhole welding state.