High-temperature superconductivity is an important subject in physics. YBCO is the first found high-temperature superconductor that is (when optimally doped) superconducting above liquid/gas nitrogen transition temperature 77K. YBCO (Yttrium- Barium-Copper-Oxide) according to the synthesis, behaves like superconducting ceramics. Depending on the Oxygen doping YBCO transitions from the antiferromagnetic insulator phase to the superconducting-metal phase. Optimally doped YBCO has superconducting transition temperature 92K and critical magnetic field Hc2=120T on 0K. In this work theoretical description of anisotropic superconducting ceramics is described aka Lawrence-Doniah model based on Ginzburg-Landau theory. Loram model of ”pseudogap” and his empirical description of YBCO magnetic susceptibility is described. We have layout the methods of cryostat magnetic measurements (SQUID and piezolever method). The magnetic measurements of YBa_2Cu_3O_6.4 and YBa_2Cu_3O_6.5 measured by SQUID method are laid out in this work. The same measurements have been measured after synthesis and sometime after synthesis in the same conditions magnetic field and temperature (B,T). Therefore, in this work, it is shown how does aging affects the superconducting properties of YBa_2Cu_3O_6.4 and YBa_2Cu_3O_6.5. At the end of this work, the magnetic measurements of YBa_2Cu_3O_6.4 and YBa_2Cu_3O_6.5 and are compared.