In order to promote the grain growth of ultrathin MgO barrier deposited on a CoFeB layer, in situ infrared (IR) heat treatment just after the deposition of MgO barrier was examined. In case that IR heat treatment was not applied, tunneling magnetoresistance (TMR) ratio of CoFeB/MgO/CoFeB magnetic tunnel junction (MTJ) was significantly decreased with decreasing resistance area (RA) product to less than 10 Ω μm2. On the other hand, TMR ratio of 206% was achieved at the RA product of 2.1 Ω μm2 when the IR heat treatment was applied. According to the cross sectional transmission electron microscope images for the samples with 0.76-nm-thick (∼4 ML) MgO barrier, the (001) oriented well crystallized structure with smooth interface was observed for the IR heated sample. Moreover, it revealed that the lateral grain size of MgO was significantly enlarged compared to that for the sample without IR heating. The improvement of TMR properties at low RA product region by the heat treatment might be due to the decrease in grain boundaries of MgO layer where the coherent tunneling of Δ1 electrons is not restricted.