Studies on factors affecting critical current density of high-Tc oxide superconductors of Y-Ba-Cu-O, Bi-Sr-Ca-Cu-O and Tl-Ba-Ca-Cu-O systems are reviewed. Although the Ho2 of these materials are much higher than those of conventional metallic superconductors, it is highly anisotropic, as can be expected from an examination of their crystal structure. The superconducting current path in these materials is also limited in CuO2 layer which is parallel to the c-plane. In addition to this anisotropic nature, the transport current flow is suppressed at the boundaries which behave as barriers of Josephson junction (weak link). This causes a rapid decrease of Jc at low magnetic field. These problems of anisotropy and weak link can be overcome by the development of processes such as Ag-sheathed tape which produce grain allignmen. In magnetic fields above Ho1, pinning strength become dominent as a factor affecting Jc. Strong pinning force has been obtained in the melt processed Y-Ba-Cu-O. However, the pinning force at 77K in Bi-Sr-Ca-Cu-O and Tl-Ba-Ca-Cu-O is still much lower than those practical levels. The weak pinning also produces a problem of flux creep. Further developments in fabrication processes that can overcome various problems limitimg transport Jc are required.
