A nanocrystalline La_(0.2)Pb_(0.4)Sr_(0.4)FeO_(3) with an average grain size about 22.3 nm was prepared by a sol-gel method.The complex crystal is of perovskite phase with orthorhombic structure.The conductivity and gas sensitivity of La_(0.2)Pb_(0.4)Sr_(0.4)FeO_(3) were investigated.Results demonstrate that the prepared La_(0.2)Pb_(0.4)Sr_(0.4)FeO_(3) shows a p-type semiconductor and has the best sensitivity to formaldehyde at an operating temperature of 175 ℃.