The mechanism of cyclooctane metathesis using confinement effect strategies in mesoporous silica nanoparticles (MSNs) is discussed by catalytic experiments and density functional theory (DFT) calculations. WMe6 was immobilized inside the pores of a series of MSNs having the same structure but different pore diameters (60, 30, and 25 Å). Experiments in cyclooctane metathesis suggest that confinement effects observed in smaller pores (30 and 25 Å) improve selectivity toward the dimeric cyclohexadecane. In contrast, in larger pores (60 Å), a broad product distribution dominated by ring contracted cycloalkanes was found. The catalytic cycle and potential side reactions occurring at [(≡SiO−)WMe5] were examined with DFT calculations. Analysis of the geometries for the key reaction intermediates allowed us to rationalize the impact of a confined environment on the enhanced selectivity toward the dimeric product in smaller pores, while in large pores the ring contracted products are favored.