Mechanical attrition of intermetallic compound TiNi powder was carried out in two different ball mills and as a function of milling temperature. The microstructural changes with milling time were followed by x-ray diffraction, TEM, and DSC. The more energetic Spex shaker mill provided a higher degree of lattice strain and rapidly refined the grain size to the nanometer size regime. Amorphization was observed in the Spex mill with a linear increase in the milling time for amorphization with increasing milling temperature. No amorphization was observed in the less energetic vibratory mill, and the grain size saturated to a constant value of 15 nm after ≥60 h of milling. A critical grain size for the amorphization of 4–5 nm was estimated from the temperature dependent studies in the Spex mill. The grain boundary energy (706 mJ/m2), estimated from the vibratory mill experiments, and the above critical grain sizes (5 nm) for amorphization were used to calculate the enthalpy supplied by the nanocrystalline grain boundaries. The calculated value of 4.1 kJ/mol was comparable to the measured enthalpy of crystallization of 3.2 kJ/mol. It is concluded that the nanocrystalline grain boundary energy is responsible for driving the crystalline-to-amorphous phase transformation induced by mechanical attrition in TiNi.