As the miniaturization of multilayer ceramic capacitors (MLCCs) is a major trend, well-fabricated ferroelectric materials such as barium titanate (BaTiO₃, BTO) with small grain and well crystallinity are increasingly in demand. Compared with other synthetic routes, solid-state reaction (SSR) is a more feasible route to prepare BaTiO₃ nanoparticles without lattice imperfections. In contrast to other researches about SSR that mainly focus on exploring improved processes to obtain BaTiO₃ nanoparticles with high tetragonality, investigations on the phase, microstructural transformation and reaction mechanism of BaTiO₃ nanoparticles were made in this study. Experiments results reveal that the reaction mechanism of forming tetragonal BaTiO₃ via SSR can be ascribed to two main reaction stages, including the formation of cubic BaTiO₃ and the cubic-tetragonal transformation of BaTiO₃. Finally, BaTiO₃ powders with a tetragonality of 1.0086 (c/a) and average particle size of 180 nm was acquired after optimizing the calcination parameters.