The influence of nickel metal distribution on the properties of Ni/Y-zeolite catalysts for CO hydrogena-tion was investigated. The studies using TPR/TPO, TEM, X-ray line broadening and FMR have shown the existence of a bidispersion of nickel metal particles; i.e., small particles were restricted inside the zeolite pores, and large particles were formed outside the zeolite crystal. CO hydrogentation reaction was performed in a differential reactor operating at atmospheric pressure, 270℃ and H₂/CO ratio of 3. Nickel metal distribution affected the activity and hydrocarbon prod-uct distribution significantly. With increasing fraction of nickel metal inside the zeolite pores the activity continued to decrease, while the production of the long chain and olefinic hydrocarbons was enhanced significantly. Especially when all the nickel metals were restricted within the pores of zeolite, a bimodal hydrocarbon product distributions for C₁ and C₄ were observed. This was ascribed to the dimerization of the primarily formed ethene to butene on the active species of Ni⁺ cations.