The REBa 2Cu 3O 7 family has similar T c and structural parameters, but with a little difference both in T c and lattice parameters. When substituting with other rare earth elements for yttrium, e.g. Ho doping in YBCO, there should exist lattice mismatch between HBCO and YBCO unit cells nearby, or among small grains, because of the different lattice parameters, especially in the c-axis. It could induce the stress field around the interface. The stress field induced by lattice mismatch will lead a free energy gradient in coherence length region, which could play a role as pinning centre as that in conventional superconductors. Using the effective defect model, the pinning force has been estimated. For confirming the idea and model, Ho-doped, Gd-doped YBCO thin film, the Ho-doping MTG YBCO sample have been prepared, which shows the critical current is enhanced. Positron annihilation and other physical analyses have been done to identify the flux pinning from the stress field induced by lattice mismatch. Different arguments will be discussed in this report also.