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Flux pinning force and irreversible line of the melt-textured-growth (MTG) YBa2Cu3O7−y (Y-123) crystals with various sizes of secondary phase particles have been investigated by measuring the magnetization of the samples under a series of magnetic fields up to 55 kOe. The magnetic Jc and the pinning force are found to increase as the particle size decreases. A linear relationship between Jc and 1/D has been obtained where D is the average size of the precipitates. The highest Jc value has reached 4×104 A/cm2 at 77 K and 40 kOe in a sample with a high density of the precipitates in nanometre size. Two peaks are observed in the Fp(H) curve in which the position of the first peak is independent of the particle size. The height and the location of the second peak is sensitive to the particle size, indicating that the peak is associated with the small particles.