Silica-supported tantalum oxide catalysts (Ta oxide/SiO2 (ALK)), prepared by chemical reaction between tantalum alkoxide and surface hydroxyl groups of SiO2, exhibited high catalytic efficiency for the vapor phase Beckmann rearrangement of cyclohexanone oxime to caprolactam. The oxime conversion reached 96.5%, and the selectivity for caprolactam was 97.5% at 573 K. The effects of precursors of tantalum oxide, physical properties of catalyst carriers, and preparation conditions were studied, and the factors in catalytic efficiency were discussed. SiO2 was the best carrier for the reaction among SiO2, Al2O3, TiO2, MgO, and SiO2-Al2O3. The pore structure of silica carriers, especially the high ratio (>90%) of the pore volume for fine pores with diameters from 4 to 200 nm to total pore volume was effective. Ta2O5/SiO2 (IMP) catalyst, prepared by impregnation of silica with an aqueous TaCl5 solution and hydrated tantalum oxide (Ta2O5 . nH2O) were less active and less selective. A large number of acid sites of intermediate strength, measured by NH3-TPD method, existed on Ta2O5/SiO2 (ALK) and were effective for the Beckmann rearrangement. On the contrary, strong acid sites appeared on Ta2O5/SiO2 (IMP). The difference in the acid property and catalytic performance between Ta2O5/SiO2 (ALK) and Ta2O5/SiO2 (IMP) was assumed to be related to the difference in morphology of tantalum oxide that was observed by SEM and EPMA. Good initial activity and high selectivity for caprolactam were observed over boron oxide supported on alumina catalyst. However, the oxime conversion declined significantly. Although the NH3-TPD profile of B2O3/Al2O3 catalyst was similar to that of Ta2O5/SiO2 (ALK), tantalum oxide of Ta2O5/SiO2 (ALK) was more stable than boron oxide of B2O3/Al2O3 in the course of reaction. The reaction between tantalum alkoxide vapor and the surface hydroxyl groups of SiO2 was observed by FTIR diffuse reflectance spectrometer. A new hydroxyl group appeared on Ta2O5/SiO2 (ALK) catalyst. The unique acid property and catalytic performance of Ta2O5/SiO2 (ALK) were discussed in connection with the new hydroxyl group.