Abstract: Different from the relationship of heat input and welding speed at melting welding, which submits inverse ratio, the relation is quite complex for friction stir welding. This paper studies the relation of welding speed and heat input at aluminum alloy friction stir welding based on thermogenesis of friction and plastic deforming. The result shows that welding speed and heat input relationship is nonlinear and shows a complex shape, which means welding speed, depending on various ranges of parameter, contributes variably to heat input. When rotary speed to welding speed ratio is constant, with the increase of welding speed, heat input and the mechanical behavior of the joint decreasing is not linear. Thus, heat input should not be measured by rotary speed to welding speed ratio.