A multi-frequency antenna comprises a first conductor, a second conductor, a grounding member, and a third conductor. The first and second conductors are respectively arranged on a first plane and a second plane. The grounding member is arranged on a third plane existing between the first and second planes. The third conductor is connected with the first conductor and arranged on the first plane also. The first and third conductors are respectively coupled to the radiated signals of the second conductor to form a first electrical path and a second electrical path. The first electrical path and the second electrical path have a phase difference of 180 degrees. The present invention features the additional third conductor. The third conductor and the first conductor are coupled to the radiated signals of the second conductor to generate opposite-phase signals. Thus are counterbalanced the interferences among the antenna systems of an identical frequency band.