Based on previously developed melt processing methods,Y2BaCuO5(211) precipitates with various size were obtained inYBa2Cu3Ox(123). In particular, a wide distribution of 211 size from 100 Å to 10 &mgr;m has been found to exist in the matrix of 123, and strongly effected the critical current density,Jc.An enhancement ofJcwas always found to be associated with the presence of extremely fine 211 precipitates. The flux pinning by these precipitates can be understood by considering the interaction of flux lines with cavities. It was found that although the flux lines are strongly pinned by cavities, the effectiveness decreases with the increasing number of flux lines trapped by them. ©1997 American Institute of Physics.