Shape-designed polyaniline (PANI) nanomaterials (nanoparticles, nanorods, and nanofibers) are prepared by adjusting the amount of the oxidizing agent and the monomer during chemical oxidation polymerization. The charge-transport properties of the precisely controlled PANI geometries at the nanometer scale were systematically investigated to identify the optimal sensing conditions to detect the nerve gas agent dimethyl methylphosphonate (DMMP). Intrinsically, the aspect ratio of PANI nanomaterials can change with the oxidation state, which is closely related to the doping level and conjugation length. Our results suggest that the transport behavior of the nanomaterials is highly dependent on their aspect ratios. Extrinsically, PANI nanomaterials deposited onto gold-interdigitated microelectrodes are able to form stable conductive channels by minimizing the contact resistance between the microelectrodes and the nanomaterials. High-performance chemiresistive sensors based on PANI nanomaterials were successfully fabricated and their sensing properties were demonstrated. The real-time response of a DMMP-sensor based on PANI nanofibers was better than that of sensors based on PANI nanoparticles or nanorods. High-performance chemiresistive sensors with a low minimum detection level (MDL, 5 ppb) could be designed through comparative studies of charge-transport properties.