Barium titanate powder plays an important role in versatile applications of ferroelectric materials. However, the barium titanate nanosized powder is difficult to be obtained through a conventional high-temperature solid-sate reaction. In this study, the barium titanate nanosized powder was synthesized by the solid-state reaction with surfactant assistance. The experimental results indicate that the mixing homogeneity between BaCO3 and TiO 2 can be improved by the addition of polyethyleneimine (PEI), a cationic polyelectrolyte, as a surfactant. The intimate contact between BaCO3 and TiO2 particles was suggested to enhance the interfacial reaction and the subsequent diffusion process. In addition, using BaCO3 with small size can further facilitate the solid-state synthesis at lower temperatures. As a result, nearly pure BaTiO3 product with particle sizes less than 50nm was prepared at 800°C for 4h.