In this paper, without using external power and active components, a design of pin coupled functional antenna tuner is presented. The tuner consists of two parts, a coupling pin and a tuning circuit. It is used to tune the bandwidth and antenna gain of the proposed slot patch antenna. The prototype, including a slot patch antenna and the tuner, was constructed and excited through a T-shape microstrip feed circuit resonated at 2.6GHz. The impedance bandwidth BW (i10dB return loss) of the slot patch antenna without coupling to the tuner was 3% referred to the operation frequency at 2.6GHz. When the tuner was matched with the impedance 75› through the coupling pin to the proposed antenna, the BW of the antenna was increased to 11% operated at 2.6GHz. However, if the tuner was matched with the impedance 25› to the proposed slot patch antenna, the impedance bandwidth of the antenna was increased 21% at operation frequency. Relatively uniform antenna gain was obtained when the matching impedance was decreased from 75› to 20›. In the meantime, the lower matching impedance corresponds to more reducing cross-polarization of the proposed slot patch antenna can be observed in the measured fleld patterns.