The present invention relates to a hair extension and a method for applying the hair extension, wherein artificial hair is attached to real hair to change a hair style without making the real hair grow or dyeing the real hair; the artificial hair is tightly attached to the real hair without using an adhesive or the like; and a state that the artificial hair attached to the real hair apparently looks natural. According to the present invention, the hair extension includes: a first artificial hair unit formed by attaching and fixating a plurality of strands of the artificial hair; a second artificial hair unit formed to face the first artificial hair unit to maintain a certain gap from the first artificial hair unit; a first joining string which is joined and fixated between the first artificial hair unit and the second artificial hair unit; and a second joining string which is located to be separated from the first joining string and is joined and fixated between the first artificial hair unit and the second artificial hair unit. The first joining string is joined to the second joining string at a part close to the second artificial hair unit to form a first knot. The second joining string is moved through the first knot in the direction of one side or the other side. The second joining string is joined to the first joining string at a part close to the first artificial hair unit. A gap between the first artificial hair unit and the second artificial hair unit can be adjusted by moving the second joining string in the direction of one side or the other side through the first knot.
