Solid state reactions induced by mechanical alloying of an elemental powder mixture of V and Si with a composition of V75Si25 were investigated using X-ray diffraction and scanning and transmission electron microscopy. It was found that the phase transitions in this system are closely related to the milling intensity. High-energy milling mainly causes the formation of intermetallic compounds, whereas low-energy milling leads to amorphization. The thermodynamics and kinetics related to the different pathways of V-Si phase transition are briefly discussed in this paper. The effect of air contamination on phase transition is also addressed.