In an antenna structure having a serial connected capacitance effect, mainly a metallic planar antenna is provided thereon at least with a first metallic plane board, and a second metallic plane board being close to but not connected to the first metallic plane board to form the effect of capacitance in serial connecting. And more, the antenna structure further has an extension arm made from a microstrip extended from the antenna or the second metallic plane board, and can be optionally grounded or not grounded, for the purpose of adjusting the impedance value of the antenna structure.