This study was carried out to investigate the effect of trans-cinnamaldehyde and high pressure treatment on milk. Cinnamon oil milk was manufactured by high speed homogenization (3,000 rpm) and high pressure homogenization (500 and 2,000 bar) processing UHT milk and trans-cinnamaldehyde of various concentrations (0 to 0.1% (w/v)). Cinnamon oil milk was inoculated with Escherichia coli (6.4 Log CFU/mL) and kept at 7 o C for 10 d to observe the antibacterial effect. The cinnamon oil milk containing 0.05% (w/v) trans-cinnamaldehyde initially began to show an antibacterial effect and Escherichia coli completely died in cinnamon oil milk added 0.1% (w/v) trans-cinnamaldehyde on the 6th day of storage. The result of the TBA value showed that the addition of 0.1% (w/v) trans-cinnamaldehyde was also effective to protect lipid oxidation. In the physical properties of cinnamon oil milk, particle sizes were enlarged in all samples during storage periods and the total color difference of cinnamon oil milk was slightly increased as level of high pressure. The surface tension of cinnamon oil milk treated 2,000 bar was remarkably higher than other samples. It seems that trans-cinnamaldehyde showed antibacterial activity and antioxidation effect at 0.05 and 0.1% (w/v) of concentration. Remarkably, high pressure treatment did not influence its microbial property but slightly affected the physical properties of cinnamon oil milk.