A structural transition from rhombohedral to cubic phase occurred in the lightly doped La1-xSrxMnO3 under high temperature and high pressure. The samples changed from an insulated (polaron ordering) state to metallic state after the phase transition. At the same time, the long-range ferromagnetic ordering of the spins of localized Mn ions collapsed into the spin glass state with only short-range ordering when the temperature decreased. The change of properties was due to the variation of Mn—O—Mn bond angle and Mn—O bond length during the structural transition.