The critical current density (J c ) value increases along the growth direction from the top to the bottom of the Y Ba Cu O samples prepared by a melt process with a unidirectional growth technique. The volume fraction of Y 2 BaCuO 5 (211) particle is constant all over the sample, while the average size decreases along the growth direction from the top to the bottom. The dependence of J c on the volume fraction and the particle size agrees with the theoretical result. It seems that an increase of J c along the growth direction from the top to the bottom can be attributed to the decrease of the 211 average size. The cause of the inhomogeneity for the 211 particle size is discussed.