Impeller and fan

An impeller, comprising:

• a hub, including a shaft;
• and
• a plurality of blades surrounding the hub, wherein each blade includes a leading edge, an outer edge, a middle portion and a trailing edge in the rotation direction of the impeller, wherein the leading edge, the outer edge and the trailing edge are around three sides of the middle portion, wherein in radial direction, the upper surface of the blade near the outer edge includes a groove structure, and the lower surface of the blade relating to the groove structure includes a peak structure,
• wherein 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, a depth of the groove structure varies from the leading edge to the trailing edge, a height of the peak structure varies from the leading edge to the trailing edge, and variations of the groove structure and the peak structure are continuous and gradual on the upper surface and the lower surface.