Co-Fe-B/MgO/Co-Fe-B magnetic tunnel junctions were fabricated using UHV magnetron sputtering. Magnetoresistance and spin-transfer switching properties were investigated as a function of Co-Fe-B free layer thickness, between 1.5 nm and 3 nm. The intrinsic switching current and the thermal stability were derived from the pulse duration dependence of the switching current, analyzed based on the thermally activated switching model. Both switching currents, corresponding to parallel (P) to antiparallel (AP) (Ic0 +) and AP to P (Ic0 –) magnetization reversal, were found to be roughly proportional to the free layer thickness. The averaged intrinsic switching current density Jc0 av = (Ic0 +–Ic0 –)/(2A) (where A is the cell area) was in the range of 1–2×107 A/cm2. The experimental values of Jc0 ± agreed with theoretical values, determined taking into account the spintransfer efficiency for the case of magnetic tunnel junction. The thermal stability of the P and AP states was different, but roughly proportional to the free layer thickness in both cases. We attribute this difference to a disparity in the net magnetic field acting on the free layer magnetization in the P and AP states. The average of the thermal stability in the two states varied from 30 to 60 when the free layer thickness was increased. According to our findings, to guarantee the non-volatility of an MRAM device for about 10 years, the Co-Fe-B free layer should be thicker than 2 nm.