We conducted a detailed comparative study of thermal stability properties of magnetic tunnel junctions (MTJs) with Ni81Fe19, Co90Fe10, and synthetic antiferromagnetic (Syn-AF) free layers. The properties were investigated using the junction magnetoresistance of current-perpendicular MTJ devices with a word line as probes. The observed sweep-rate-dependent coercivities were analyzed using the Sharrock formula. The results confirmed that the properties of KuV/kBT values in the MTJs with the Ni81Fe19 free layer is mainly determined by the shape anisotropy, which is not true in the MTJs with the Co90Fe10 and Syn-AF free layers. The study also confirmed that the effective volume of the Syn-AF free layer increased due to the interlayer coupling and that the shape anisotropy in the Syn-AF free layer was less than that in the mono-free layer.