Abstract Aligned poly(methyl methacrylate) (PMMA) fibers were fabricated by electrospinning and then nanostructured polyaniline (PANI) was grown on the surface of the parallel PMMA fibers via in situ solution polymerization. The structure and morphology of the aligned PANI/PMMA composite fibers were characterized by scanning electron microscopy (SEM) and Raman spectrometry. The gas sensing properties of the composite fibers were investigated under ultra-low ammonia concentrations at room temperature. It is found that the arranged fibers exhibited high sensitivity and fast response upon exposure to ammonia vapor of 1–30ppm. The normalized resistance R/R0 increased linearly with the ammonia concentration increasing. The sensing mechanism was also discussed. The results indicate that aligned PANI/PMMA composite fibers are promising candidate for fast detection of toxic ammonia gas.