An impeller includes a hub and blades. In radial direction, an upper surface of the blade near the outer edge includes a groove structure, a lower surface of the blade relating to the groove structure includes a peak structure, the lower surface of the blade includes a recess structure aside the trailing edge or the leading edge. The recess structure is connected to the peak structure. The blade has at least five airfoils from the inner edge to the outer edge. The blade is defined by continuously connecting the at least five airfoils at different sections in sequence so as to form the groove structure and the peak structure respectively on the upper surface and the lower surface. The groove structure and the peak structure are aside the outer edge of the blade.