A new planar microstrip balun with two operation bands is presented and investigated in this letter. Originated from the traditional Marchand balun, the proposed dual-band balun is designed on a symmetrical four-port network with one short-circuited port. Derived equations and design charts show that the frequency ratio (m = f(2)/f(1)) of these two bands could be flexibly controlled in a wide range of 2.0-4.6. Moreover, inherent transmission zeros nearby each band have been introduced into this dual-band balun, aiming to improve the frequency selectivity. For verification, a prototype dual-band balun operating at 2.4 and 5.2 GHz (m = 2.17) is implemented and measured. Both simulation and measured results are recorded with good agreement.