In this thesis, high temperature superconductors Y 1-x Co x Ba 2 Cu 3 O 7-? samples with were prepared by the conventional solid-state reaction. The x-ray diffraction pattern analysis shows that, all the samples until x=0.008 were single phase and no impurity phases are detected, by increasing of doping from x=0.008 the BaCuO 2 impurity phase is detected in samples that its uncreasing by enhancing the dopping content. All samples have an orthorhombic structure with pmmm symmetry. Lattice parameters and unit cell volume of samples were found by Rietveld method and it shows that lattice parameter c increases with increasing Co. also, in-plane Cu(2)-O(2) bond length were found by Rietveld method and it is seen that Cu(2)-O(2) bond length decrease with increasing Co up to optimal content of 0.02. in the other word, CuO 2 planes become . charge carriers have better chances for transfering so it can be strong evidence for increasing of transition temperature yp to this concentration. By increasing concentration from optimal content 0.02, Cu(2)-O(2) bond length starts to increase. It shows that CuO 2 planes become flat and this describes decreasing of transition temperature by increasing of concentration after 0.02. the real part of magnetic susceptibility versus samples’ temperature shows sharp drop from normal state to diamagnetic- superconducting state, that it’s because of in grain transtion. By anlaysing of susceptibility data we see that by increasing of magnetic element of Co till 2% doping, the transition temperature of superconductivity increases. That increasing of holes by substitution of Co with oxidation of +2 and +3 in the place of Y with +3 oxidation can be the reason for icreasing in transition temperature. Then by increasing of doping concentration, transition temperature starts to decrease that overdoping of holes concentration after x=0.02 can be the reason of this deacrising of transition temperature. Also, this increasing of transition temperature till 0.02 and then deacrising of transition temperature can be seen in imaginary part of magnetic susceptibilty curve. Also, altitude of imaginary peaks in susceptibility curve, increase by increasing Co till 0.03, that it show the inhancement of flux pinning up to 0.03. then by decreasing of altitude and width of peaks after x=0.03 flux pinning become decrease. The electrical resistivity show nearly linear temperature dependence in the normal state and by increasing of Co content it shows a better metallic behavior and resistivity of samples deacrese till 0.02. by increasing of doping concentration resistivity increases. Electrical resistivity curve shows an increase in transition temperature till 0.02 and then by increasing of Co, critical temperature decrease. Increasing of transition temperature with increasing of Co element in Y site till an optimal concentration is in contradiction with Abrikasov-Gorkov theory. Critical current density were measured for different doping concentration. It shows that critical current density increase till x=0.03 and it gets to it’s higher point at x=0.03 and by increasing the doping content after this, critical current density starts to decrease and it gets to its minimum point at x=0.07. it seems that cobalt element acts like center of flux pinning in compound and improves the critical current density.