A microampere dc current is measured by a fiber optic sensor. The phase change of the guided light induced by ohm's heat over a golden-coated fiber is detected by an interferometer. The thermal conduction of the optical fiber and the relation of the temperature-induced phase change of the guided light are explored. A Michelson interferometer plus a Faraday rotator mirror enable system immunity from the polarization-induced signal fading due to the environmental disturbance, especially by vibration and temperature. A passive phase-generated carrier (PGC) demodulation scheme enables the system to demodulate the interference signal stably by shifting the signal away from the high-noise, low-frequency band through a carrier. The sensitivity of the sensing system is improved by a biased method. The minimum measurable dc current is 3.416×10–7 A with a dynamic range of 89.33 dB.