This paper describes a new low-power write driver circuit for mobile hard disk drive preamplifiers. To achieve low power consumption, we developed a write driver circuit with a single-stage MOS transistor as the current driver, which both switches and controls the write current. We also developed a reflection cancellation method to suppress the distortion of the write current waveform during write transition. Fabricated using 0.35-µm SOI-BiCMOS technology, this write driver circuit consumes low power, 380 mW (at 100 MHz).