Superstructure orthorhombicII (OII) phase in polycrystalline samples of YBCO_x was observed by using synchrotron in-situ high resolution powder diffraction (HRPD) techniques at 300˚C. Rietveld method (RM) refinement was employed to analyse the collected diffraction data under proper oxygen partial pressure atmospheres. Our study indicates that by oxygen uptake, the samples undergo structural phase transition from initial tetragonal phase to the superstructure OII phase with x ≈0.6. According to our previous study this transition follows 1^st order Gibbs lever role. Evidence for exsistence of dominated OII superstructure in powder samples at 300 ˚C, makes a valuable correction on the YBCO_x’s structural phase diagram, which has been historically controversial and also would be interesting for applications.