The compound [ThCu3] (Mn4)O12 belongs to the series of [AC3] (B4) O12 perovskite-like oxides in which the icosahedral sites [A] are occupied by the Th4+ cations, the square planar sites [C] by the Cu2+ cations, and the octahedral sites (B) by the Mn3+ and Mn4+ cations in equal proportion. Previous measurements indicated a ferrimagnetic behavior with an ordering temperature TN= 430 K, a spontaneous magnetization of 9µB/formula unit at 4.2 K and a magnetic transition at Tt = 180 K. The ferrimagnetic structure, as determined from powder neutron diffraction data, is consistent with the values of magnetic interactions that can be estimated from magnetization and susceptibility measurements. The low temperature transition has been studied by magnetization measurements and X-Rays techniques. It seems to be of second order with anomalies in the cubic cell parameter and of the magnetic moments. No changes in cristallographic symmetry and magnetic structure are observed