The activity of electrochemical oxygen reduction (EOR) for the pyrochlores Ln2Ru2-XO7-δ (Ln=Pr,Nd,Sm) [LnR] were examined in 0.1 M KOH aqueous solution at 70oC. The onset voltage (Vo) of the oxygen reduction current and the efficiency (E4) of 4-electron reduction of oxygen were evaluated by semi-steady state voltammetry with rotating ring-disk electrodes. PrR with the highest EOR activity showed Vo = ~ 0.85 V vs. reversible hydrogen electrode and E4 values above 80 %. Their Vo and E4 values show that LnR containing Ln with a smaller atomic number has a higher EOR activity, i.e. the order of the activity is PrR > NdR > SmR. This was in good agreement with that of the lattice parameters of LnR. These results indicate that the EOR activity of LnR depends on the kind and/or the size of the lanthanide metal ion on the A-site. PrR and NdR exhibited higher E4 values than known excellent Pb2Ru2O7-δ electrocatalyst containing toxic Pb.