In this study, the effect of synthesis conditions and annealing process on the sensitivity and stability of gas sensors made of flame-synthesized Zn-doped γ-Fe2O3 particles was investigated. Zn-doped γ-Fe2O3 particles were synthesized by flame spray pyrolysis using either H2/Air or H2/O2 coflow diffusion flames. The particles were then annealed at 325~350˚C in a tube furnace under air atmosphere. Both as-synthesized and annealed particles were used as gas sensing materials to construct gas sensors. Transmission electron microscopy (TEM), X-ray diffraction (XRD), Brunauer-Emmett-Teller surface area measurement (BET), Williamson and Hall (WH) method were employed to characterize the particles. Gas sensors were fabricated by applying the as-synthesized and annealed particles on interdigitated electrodes. The response of the gas sensor to acetone vapor, H2 in dry synthetic air was measured before and after three days of aging. High-temperature flame (H2/O2) generated nanometer-sized particles; lower temperature flame (H2/Air) generated micrometer-sized particles. Fe2O3 particles doped with 15% Zn showed the highest sensitivity. The sensors made from as-synthesized particles showed a gas sensing sensitivity that was 20 times higher than the literature value. The sensors made of microparticles lost their sensing ability after three days of aging, but sensors made of nanoparticles did not show significant change after aging. Sensors made of annealed particles (either micro or nano) did not have significant gas sensing ability, but annealing process improved the stability of gas sensors. Analysis using the WH method showed that the microstrains decreased significantly in both H2/O2 and H2/Air flame particles after annealing. The results showed that sensors made of nanoparticles have higher gas sensing signal, and more resistant toward aging than sensors made of microparticles. In addition, annealing process affected on the stability favorably due to reduction of structural defects.