Developing alternative oxygen reduction reaction (ORR) catalysts to replace precious Pt-based metals with abundant materials is the key challenge of commercial application of fuel cells. Owing to their various compositions and tunable electronic properties, transition metal dichalcogenides (TMDs) have the great potential to realize high-efficiency catalysts for ORR. Here, various 3R-phase dichalcogenides of group VB and VIB transition metals (MX₂, M = Nb, Ta, Mo, W; X = S, Se, Te) are investigated for ORR catalysts by using density functional theory calculations. The computed over-potentials of group VB TMDs are much less than those of group VIB TMDs. For group VB TMDs, a volcano-type plot of ORR catalytic activity is established on the adsorption energies of *OH, and NbS₂ and TaTe₂ exhibit best ORR activity with an over-potential of 0.54 V. To achieve even better activity, strain engineering is performed to tune ORR catalytic activity, and the minimum over-potential of 0.43 V can be realized. We further demonstrate that the shift of p orbital center of surface chalcogen elements under strain is responsible for tuning the catalytic activity of TMDs.