In situ transmission electron microscopy observation revealed that BaGd2Mn2O7 undergoes several phase transitions upon heating. The orthorhombic Pnnm phase (I) at room temperature was transformed into a tetragonal P4(2)/mnm (II) at around 420 K via an intermediate phase. On further heating, the other phase (III) nucleated in the phase II matrix at 623 K and grew until covering the whole inspected region by 723 K. Phase III has a 1.5% lattice mismatch along c compared with phase II, and presumably has a monoclinic symmetry. The transition between II and III was of the first order.