This communication proposes a frequency-reconfigurable patch antenna using low-loss transformer oil (epsilon(r) = 2.2), which is a kind of high-frequency liquid. A two-layer substrate is introduced between the patch and ground plane. The upper one is made of homogeneous polytetrafluoroethylene (epsilon(r) = 2.2) substrate, while the lower layer consists of a transparent container, including air and transformer oil layers. By varying the height of transformer oil layer, the volume ratio of air to liquid is changed as a function of this height. As such, the effective permittivity of the entire substrate underneath the patch can be tuned so as to realize the so-called frequency reconfiguration. After extensive analysis is carried out, a prototype of the proposed antenna is fabricated and measured. Measured results are found in reasonably good agreement with the simulated ones. A wide tunable impedance bandwidth of 32% from 1.42 to 1.96 GHz is achieved. Moreover, the prototype shows high radiation efficiency because of the low-loss transformer oil.