The microstructural evolution and formation mechanism of nanograins in Ti-2Al-2.5Zr α-titanium alloys and Ti-10V-2Fe-3Al metastable β-titanium alloys subjected to multi-impact process at ambient temperature were investigated using optical microscopy, X-ray diffraction and transmission electron microscopy. The results show that deformation twinning, dislocation activities and shear banding successively dominate plastic deformation in Ti-2Al-2.5Zr alloy. In contrast, deformation-induced martensitic phase transformation becomes prevalent in Ti-10V-2Fe-3Al alloy. Martensitic subdivision, shear banding and reverse martensitic transformation contribute to grain refinement. Moreover, nanograins are finally achieved in both titanium alloys at a strain of 1.2, but grain refining efficiency is more significant in Ti-10V-2Fe-3Al alloy. The accelerated grain refinement could be attributed to the assistance of martensitic phase transformation.