A fan comprising: a hub ; and a plurality of blades extending from the hub , wherein each of the plurality of blades comprises: a blade front end having a predetermined curvature; a blade side end extending from the blade front end for forming the lateral side of the blade ; and a blade rear end extending from the blade side end , wherein the blade rear end comprises an alternating pattern of ridge parts and groove parts , the pattern extending along the edge of the blade rear end , wherein each of the plurality of blades has a positive-pressure surface and a negative-pressure surface opposite to the positive-pressure surface , wherein the ridge parts protrude from the positive-pressure surface and the groove parts protrude from the negative-pressure surface , wherein the alternating pattern comprises: a first upper end part defined between a first ridge part and a second ridge part ; a first lower end part defined between a first groove part and a second groove part , the first lower end part being next to the first upper end part ; a first connection part extending from the first upper end part of the pattern to the first lower end part of the pattern being next to the first upper end part ; and a second connection part extending from a second upper end part of the pattern being next to the first upper end part to the first lower end part of the pattern,wherein a distance (D) between adjacent two of the ridge parts is different from a distance (C) between adjacent two of the groove parts , characterized in that the first connection part and the second connection part have different lengths. a hub ; and a plurality of blades extending from the hub , wherein each of the plurality of blades comprises: a blade front end having a predetermined curvature; a blade side end extending from the blade front end for forming the lateral side of the blade ; and a blade rear end extending from the blade side end , wherein the blade rear end comprises an alternating pattern of ridge parts and groove parts , the pattern extending along the edge of the blade rear end , wherein each of the plurality of blades has a positive-pressure surface and a negative-pressure surface opposite to the positive-pressure surface , wherein the ridge parts protrude from the positive-pressure surface and the groove parts protrude from the negative-pressure surface , wherein the alternating pattern comprises: a first upper end part defined between a first ridge part and a second ridge part ; a first lower end part defined between a first groove part and a second groove part , the first lower end part being next to the first upper end part ; a first connection part extending from the first upper end part of the pattern to the first lower end part of the pattern being next to the first upper end part ; and a second connection part extending from a second upper end part of the pattern being next to the first upper end part to the first lower end part of the pattern,wherein a distance (D) between adjacent two of the ridge parts is different from a distance (C) between adjacent two of the groove parts , a first upper end part defined between a first ridge part and a second ridge part ; a first lower end part defined between a first groove part and a second groove part , the first lower end part being next to the first upper end part ; a first connection part extending from the first upper end part of the pattern to the first lower end part of the pattern being next to the first upper end part ; and a second connection part extending from a second upper end part of the pattern being next to the first upper end part to the first lower end part of the pattern,wherein a distance (D) between adjacent two of the ridge parts is different from a distance (C) between adjacent two of the groove parts , characterized in that the first connection part and the second connection part have different lengths. <EOS>