A model of the surface roughness in orthogonal turn milling is established. The relationship between main cutting parameters and the machined surface roughness is analyzed. Turn milling experiments of Ti 6.5Al 3.5Mo 2.5Sn 0.3Si (TC9 for short) are carried out with an end milling cutter. The effects of the main cutting parameters in orthogonal turn milling， such as cutter and workpiece rotary speed ratio， axial feed and accentric， on surface roughness are studied. The results obtained in dicate that the surface roughness decreases with the increase in cutter and work piece rotary speed ratio and eccentric. However， the tendency of the surface rou ghness with respect to axial feed is contrary to the last one. Surface quality beneath Ra <1 μm can be achieved. So high quality surface can be obtained， when the rota tionally symmetrical workpieces is processed in orthogonal turn milling.