Directional solidification experiments were performed on hyperperitectic Nd13.5Fe79.75B6.75 alloy using a Bridgman type furnace. The relationship between secondary dendrite arm spacing (lambda(2)) of primary gamma-Fe phase as a function of growth velocity (V) was established. Experimental results show that 2 decreases with increasing V and give a constant value)lambda V-2(0.33) of 5 x 10(-7) m(1.33) s(-0.33). Using arm coarsening theory, secondary arm spacing of gamma-Fe dendrites was calculated, during which temperature gradient changing with growth velocity in the experiment was taken into account. The calculation gives a constant value lambda(2)V(0.31)of 8 x 10(-7) m(1.31) s(-0.31). Theoretical predictions are in reasonable agreement with experimental results. In peritectic reaction, the dissolution rate of gamma-Fe dendrite arms varied from 0.0061 to 9.1 mu m/s, as the growth velocity increased from 1 to 100 mu m/s. The dissolution rate of gamma-Fe dendrites was extremely small in low-velocity specimens with plane front peritectic interfaces. (c) 2008 Elsevier B.V. All rights reserved.