The structural and magnetic analysis of polycrystalline samples of La 2 / 3 Ba 1 / 3 MnO 3 has been carried out between 5 and 370K by means of neutron-diffraction technique and DC magnetic measurements. Below the Curie point T C =314K the system undergoes a structural phase transition R3c-Imma. The transition extends over a wide temperature range, proceeds most rapidly in the vicinity of T m =200K, and has characteristic features of a martensitic phase transition. The uniaxial pressure effect on the DC magnetization is maximal at 200K. Two magnetic phase transitions (of the second order at 250K and of the First order at 177.5K) are reflected mainly in the R3c phase crystal parameters.