Abstract: Effects of ultra high pressure processing on flavour compounds in orange juice were investigated. Samples were subjected to high pressure ranging from 100 to 500 MPa. After high pressure treatments, the flavour compounds were extracted by Solid-Phase Microextraction (SPME). Analysis was carried out by Gas Chromatography-Mass Spectrometry(GC-MS). The key flavour compounds of orange juice were three alkenes (namely limonene, myrcene and α-pinene), two alcohols (linalool and α-terpineol), three aldehydes (citral, octanal and decanal), one ketone (carvone) and two esters (ethyl butyrate and 3-OH-ethyl hexanoate). After ultra high pressure treatment, limonene concentration was decreased greatly while myrcene and α-pinene concentrations were not changed significantly and the concentration of limonene was decreased by 75% after high pressure treatment of 500 MPa for 15 min, α-terpineol and carvone concentrations were increased significantly. The aldehydes concentrations were retained well. The ester concentrations were changed under high pressure treatment, but generally their concentrations were not changed remarkably. Under ultra high pressure, limonene was converted to α-terpineol and carvone respectively via acid catalysis and oxidation reaction. Orange juice was processed under pressure above 300 MPa generally. The more losses of flavour compounds were occurred in orange juice under 500 MPa in the study. Thus, pressure of about 400 MPa was chosen as the optimum condition.