The La0.9Ba0.1MnO3-δ/SrTiO3:Nb p-n heterojunctions have been fabricated by growing the oxygen nonstoichiometric La0.9Ba0.1MnO3-δ thin film on the Nb doped SrTiO3 substrate with pulsed laser deposition. The good rectifying curves have been demonstrated in the temperature range of 20—300K. The diffusion voltage VD exhibits a maximum near the transition temperature of La0.9Ba0.1MnO3-δ thin film, which is different to the observation in the oxygen stoichiometric La0.9Ba0.1MnO3/SrTiO3:Nb p-n heterojunction. The interesting phenomenon has been well explained with the resistance-temperature experiment of La0.9Ba0.1MnO3-δ thin film and energy band calculations.