MgB 2 samples were prepared by using 96% boron ͑B͒ powder with strong crystalline phase that had been ball milled for various times. We observed samples that contained ball-milled 96% B in comparison with one made from as-supplied commercial 96% B, with the results showing a significant enhancement in the high field critical current density ͑J c ͒ due to small grain size and better reactivity. Specifically, many grain boundaries for MgB 2 could be acting as strong flux pinning centers. Based on

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... Rowell connectivity analysis, when the ball-milling time increased, the connectivity factor, described as the active cross-sectional area fraction ͑A F ͒, was decreased. This implies that the intergrain connectivity became worse. These properties could lead to poor J c in low field. However, the pinning force strength, J c 1/2 ϫ B 1/4 , of samples using ball-milled 96% B is larger than that of the reference sample using commercial 96% B powder. These results accompany enhanced irreversibility ͑H irr ͒ and upper critical fields ͑H c2 ͒.