The demand for ultra-high-temperature (UHT) processed milk is increasing worldwide. Meanwhile, the problem of destabilization during its shelf life, with age gelation and fat destabilization being the most prominent defects, is still a major factor that limits shelf life and commercial exploitation. Previous studies have revealed that the main factor of UHT milk destabilization is protein enzymatic hydrolysis. The two main categories of residual proteases in UHT milk are the native protease plasmin and heat resistant bacterial proteases, both of which are closely related with the raw milk quality. However, a complete overview of how the proteases destabilize casein micelles and fat globules in UHT milk is still lacking. Hence, this thesis explored the enzymatic factors associated with destabilization of UHT milk, aiming to provide new insights into the mechanisms of UHT milk instability and to determine the significance of raw milk quality control in UHT milk production.