Elongated mullite (Al₆Si₂O₁₃) was synthesized at 1 773 K for 5 h via an in-situ solid-reaction method using granular mullite powders as raw materials, and TiO₂ as an additive. Based on thermodynamic analysis of the reaction process, the effects of TiO₂ contents on the phase composition, phase contents and microstructure of synthesized samples were investigated, and the growth mechanism of elongated mullite was discussed. The results showed that elongated mullite with the length of 15.2 μm were formed for the sample with TiO₂ contents ≥ 7 wt%. Lattice distortion of mullite and the formation of liquid phase in the sample with addition of TiO₂ resulted in the anisotropic grain growth of mullite.