This paper reports on the design of an asymmetrical balun with a tunable transmission zero. First, a compact balun is proposed with asymmetric feeding points. The proposed balun possesses a tunable transmission zero for the harmonic suppression. The bandwidth of the proposed design can be easily regulated by the asymmetrical feeding structure. The proposed design is fabricated using ARLON 25N with thickness of 0.46 mm. The measured results agree well with the simulation. According to the measurement results, the maximum insertion loss is 1.2 dB, the maximum in-band phase imbalance is within 4 degrees, and the maximum in-band magnitude imbalance is less than 0.4 dB.