Japanese Journal of Applied Physics, 44 (2005) pp. L 1442--L 1445 We investigated dependence of tunnel magnetoresistance effect in CoFeB/MgO/CoFeB magnetic tunnel junctions on Ar pressure during MgO-barrier sputtering. Sputter deposition of MgO-barrier at high Ar pressure of 10 mTorr resulted in smooth surface and highly (001) oriented MgO. Using this MgO as a tunnel barrier, tunnel magnetoresistance (TMR) ratio as high as 355% at room temperature (578% at 5K) was realized after annealing at 325 C or higher, which appears to be related to a highly (001) oriented CoFeB texture promoted by the smooth and highly oriented MgO. Electron-beam lithography defined deep-submicron MTJs having a low-resistivity Au underlayer with the high-pressure deposited MgO showed high TMR ratio at low resistance-area product (RA) below 10 ohm-um^2 as 27% at RA = 0.8 ohm-um^2, 77% at RA = 1.1 ohm-um^2, 130% at RA = 1.7 ohm-um^2, and 165% at RA = 2.9 ohm-um^2.