In order to study the intrinsic particle size effect of Co catalysts in Fischer-Tropsch (FT) synthesis, a series of Co/SiO2 model catalysts with various metal coverages (0.25-6.0 ML), corresponding to sizes in the range of 1.4-10.5 nm were prepared in an ultrahigh vacuum chamber and tested at 513 K under atmospheric pressure in a batch reactor. The Co particle size distribution and the number of active sites were estimated from the TEM images. Kinetic data and post-reaction XPS spectra showed that for the small Co particles (1.4-2.5 nm), the Co surface was readily oxidized by water vapor, while for the relatively large Co particles (3.5-10.5 nm), such oxidation was not evident. The oxidation for the small Co particles leads to lower TOF and higher CH4 selectivity, while both reactivity and selectivity were relatively constant for the relatively large Co particles. The lack of intrinsic particle size effect for the metallic Co particles in the range of 3.5-10.5 nm is consistent with structure insensitivity in FT synthesis. (C) 2011 Elsevier B.V. All rights reserved.