In this paper, it is reported that the small size defects, which resul
t from the introductions of Y, Ba, Cu and O ionic vacancies and the pa
rtial substitutions of Ln (Gd, Sm, and Eu) for Y in YBa2CU3O7-delta sy
stem, may pin the flux lines. From the experimental results, authors s
uggest that the flux pinning effect at lower field relates to the magn
etic interaction between the vortex and the surfaces of grains in the
samples; and that the flux pinning at higher field belongs to the core
interaction mechanism in the samples with ionic vacancies and stems f
rom the elastic interaction between the vortex and the lattice stress
field in the partially substituted samples.