An ultralow-limit gas microsensor based on an ultrathin conducting oligoaniline film integrated with microscale gold electrodes is developed. A nanoscale oligoaniline film is fabricated on a poly(dimethylsiloxane) (PDMS) substrate using graft polymerization using FeCl3, a mild oxidant, rather than conventional (NH4)(2)S2O8. The as-fabricated film is around 14 nm in thickness and above 85% transmittance on a PDMS substrate with a smooth surface morphology and high conductivity. Taking NH3 as a protocol, the nanoscale oligoaniline film microsensor shows an ultralow detection limit to the ppb level with more rapid response and high sensitivity to NH3 compared to the thicker PANI film using conventional methods.