Hydrogen sensor based on pentacene organic thin-film transistor (OTFT) with palladium (Pd) source and drain electrodes and high-k HfLaO gate dielectric is fabricated. The sensor exhibits clear change in drain current when exposed to different H2 concentrations in air at room temperature. Rapid, reversible and concentrate-dependent H2 response of the OTFT is observed when the device is exposed to various H2 concentrations ranging from 200 ppm to 17,000 ppm. In a hydrogen-containing ambient, the device exhibits an obvious decrease in drain current because after absorbing hydrogen, the Pd source/drain electrodes expand to increase the contact resistance of the device. To support the physical mechanism of hydrogen response of the OTFT with Pd electrodes, a control sample with gold (Au) as source and drain electrodes is also fabricated for comparison.