High speed milling experiments using nitrogen-oil-mist as the cutting medium were carried out to in-vestigate the characteristics of chip formation for Ti-6Al-4V alloy. Within the range of conditions employed in these experiments. During the macro and micro analysis of the Ti-6Al-4V chips, an optical microscope and a scanning electron microscope (SEM) were used to study the microstrocture and the morphology of the chips, and the X-ray photoelectron spectroscopy (XPS) was employed for chemical analysis. Comparisons were made to study the influence of different cutting media (nitrogen-oil-mist, air-oil-mist and dry cutting condition) on chip formation. Results indicate that cutting media have significant effects on chip formation. Nitrogen-oil-mist is more suitable for improving the contact condition at chip-tool interface and increasing the tool life in high speed milling of Ti-6Al-4V alloy than air-oil-mist and dry cutting.