Carbon material supported bi- or tri-metallic nanoparticles were usually used to replace noble metals, such as platinum, for improving catalytic performance and reducing the cost. In this paper, a carboxylate-functionalized multi-walled carbon nanotube supported bimetallic platinum–cobalt nanoparticles catalyst was synthesized using a simple one-step ultrasonic method. Electrochemical experiments showed that this catalyst exhibited excellent electrocatalytic activity in acid solution for the oxygen reduction reaction. In detail, the onset potential and half-wave potential of this catalyst positively shifted compared with the commercial platinum/carbon catalyst. The as-prepared catalyst also presented a high mass activity. Additionally, it showed a four-electron reduction pathway for the oxygen reduction reaction and exhibited better stability (about 82.8% current density was maintained) than platinum/carbon during the current durability test.