Three Ni-based catalysts with the same nickel content (10 wt%) were prepared by conventional impregnation of commercial gamma-Al2O3 support (NiAlCO-IM), sol-gel-made gamma-Al2O3 (NiAlSG-IM) and direct sol-gel processing from organometallic compounds (NiAlSG), respectively. Their catalytic activity and coking resistivity for CO2 reforming of methane to synthesis gas were studied in a continuous-flow microreactor under atmospheric pressure. Although three catalysts had comparable activity, they showed a great difference in coking resistivity. NiAlSG-IM catalyst had excellent coking resistivity with no obvious coke observed even after 80 h of reaction on stream, under thermodynamically severe conditions (CO2/CH4 = 0.88, 700 degrees C). A little coke deposited on NiAlSG, with an average coking rate of 0.003 g(carbon) g(cat.)(-1) h(-1). However, fast and heavy coke deposition occurred on NiAlCO-IM catalyst, with an average coking rate of 0.095 g(carbon) g(cat.)(-1) h(-1), and the reaction was sustained only about 3.5 h accompanied with the plugging of reactor. NiAlSG-IM catalyst prepared from organometallic compounds possesses very high BET surface area and small metallic Ni particles. The small size of metallic Ni particles is a key factor to prevent coke formation and the critical size of Ni particles to inhibit coke deposition is suggested to be about 10 nm. gamma-Al2O3 support made from sol-gel processing of organometallic precursors may also play some role to prevent coke formation.