Cavitation inception in tip leakage flows remains a challenging topic in the engineering field, as the effect of tip gap width on inception is unclear. The present study is devoted to an analysis of the effect of gap width on tip leakage cavitation inception by using full-wetted numerical simulations. Numerical results show that the cavitation inception number is strongly related to the dimensionless gap width τ, which is defined as the ratio of tip gap width to the maximum hydrofoil thickness, and the reason behind it is explained by the specific flow structures. The cavitation inception number of suction side (SS) sheet cavitation decreases gradually with the increase of dimensionless gap. The cavitation inception numbers of tip leakage vortex (TLV) and tip separation vortex (TSV) increase first and then decrease with the increase of the gap, reaching the maximum at and , respectively. The main reason is that in the gap range of , TLV and TSV cores have the highest vorticity and the lowest pressure.