The structural study of Sm 0.2Ca 0.8MnO 3 and Sm 0.2Ca 0.8Mn 0.9Ru 0.1O 3 versus temperature has been carried out by neutron diffraction and electron microscopy in connection with the magnetic and transport properties. A structural transition from a Pnma paramagnetic insulating structure to a P2 1/m C-type antiferromagnetic insulating state at TN≈150 K is observed for the undoped phase. This low temperature monoclinic structure results from a cooperative Jahn-Teller (JT) distortion of manganese cations, hindering the appearance of magnetoresistance in Sm 0.2Ca 0.8MnO 3. In contrast, the Ru-doped phase, which exhibits two magnetic transitions at TC≈200 and TN≈110 K, keeps the Pnma structure in the whole temperature range, from 4 to 300 K, showing that Ru-doping destabilizes the JT effect. The influence of ruthenium doping upon the structure and the appearance of ferromagnetism and metallicity in Sm 0.2Ca 0.8MnO 3 is discussed by comparing its behavior with those of the x=0.90, 0.85, and 0.80 samples in the Sm 1- xCa xMnO 3 series.