In this work, Sr2+ dopant effects of Ba0.9Sr0.1TiO3 and La0.9Sr0.1CrO3-delta doped-perovskite nanoparticles on increasing proton conductivity, fuel cell performance, and mechanical and thermal stability of polybenzimidazole-based nanocomposite membranes were studied. The Sr2+ dopant creates cation vacancies in Ba0.9Sr0.1TiO3 doped-perovskite nanoparticles and oxygen vacancies in La0.9Sr0.1CrO3-delta doped-perovskite nanoparticles. The oxygen vacancies, which decrease columbic repulsion between protons and positive ions, have a more important role than the cation vacancies. They provide high surface area and high interfacial interaction between La0.9Sr0.1CrO3-delta doped-perovskite nanoparticles, phosphoric acid, and polybenzimidazole for proton transfer and increase the proton conductivity of the nanocomposite membranes. In addition, the results of relative humidity effects showed that the ordered arrangement of oxygen vacancies of the La0.9Sr0.1CrO3-delta doped-perovskite nanoparticles creates a specific pathway in the nanocomposite membranes for increasing proton transfer in the presence of relative humidity. Furtheremore, at phosphoric acid doping level of 13 mol phosphoric acid per monomer unit, proton conductivity of the nanocomposite membranes containing 8 wt.% La0.9Sr0.1CrO3-delta doped-perovskite nanoparticles was obtained as 126 mS cm(-1) at 180 degrees C and 6% relative humidity. The nanocomposite membrane showed the best performance and the power density of 0.62 W cm(-2) at 180 degrees C and 0.5 V.