The high temperature superconductor of Y -x x 2-RxBª 2- y Sr y Cu3 0σ.5o compounds were prepared, as a ceramic powder, then pressed into pellet shaped, by solid state reaction . Eu,La and Ho were used as isovalent substitution for (x) and Sr as (y), nine sample have been investigated . The preferable preparation conditions have been taken into account through the oxygen flow sintering temperature and forming pressure to obtain a good behaviour of superconductor. lodonetric titration was used t find the value of (6) from which the amount of oxygen in the sample is determined . The phase transformation of all samples were examined by three methods, firstly, by using differential thermal analysis (DTA) technique to find the temperature range of transformation of the compounds, secondly X-ray diffraction (XRD) technique and thirdly by resistivity measurements of (T ) . It was found that the DTA-peaks of transformation shifted to a lower temperature with increasing the values of (x) for the system Y−x Rx Ba2-y The (Sr y Cuз Oσ.5+σ with isovalent substitution in Ru=Eu,La, and Ho) observed transformations were from orthorhowbic to tetragonal phase with different range of temperatures . X ray diffraction and DTA-analysis for samples quenched at different quenching temperatures were investigated to allocate phase transformations . With the aid of a computer program, whose named is (LSQM-BAS), the values of lattice constants (a,b,& c) for both orthorhombic and tetragonal phases were calculated . The (XRD) and (DTA) data were in complete agreement concerning the phase transformation and of cource they are complementary