We report an anomalous phenomenon in Pt supported on thiolated multi-walled carbon nanotubes (Pt-S-MWNT): oxygen reduction reaction (ORR) activity increases with accelerated durability test (ADT) cycles. Sub-nanometer-sized Pt clusters on S-MWNT were gradually agglomerated to an optimal size with ADT cycles, and finally showed increased ORR activity after the ADT.