Abstract: The magnetization feature under high magnetised fields and the effect of magnetised field on demagnetization behaviour for Nd₄Fe_77.5B_18.5 nanocrystalline alloy were investigated. The results indicate that the alloy is not saturated magnehsed while magnetization-field μ_OH_m=7 T. The material shows typical characteristics of permanent magnet as magnetization-field H_m is larger than a critical magnetization field H_mc. The permanent magnet features of this alloy origine from exchange-coupling inteaction between crystalline grains.