The transformation and grain growth behaviour of an icosahedral phase from the supercooled liquid region of rapidly quenched Zr65Cu7.5Al7.5Ni10Ag10 metallic glass have been examined by transmission electron microscopy. The precipitation of the icosahedral phase proceeds according to a polymorphic reaction without any concentration change. The grain size increases linearly with an increase in annealing time and the growth rate strongly depends on the temperature. An Arrhenius plot of the logarithm of growth rate versus the reciprocal of annealing temperature yields a linear relationship, from which an activation energy for grain growth of 409 kJ mol-1 is obtained. This activation energy is much higher than those in the Al-Cu-V and Al-Mn-Si systems. This activation energy is attributed to the strong bonding among constitutional elements and the necessity of the rearrangement of atoms on a long length scale for grain growth. The kinetic data suggest that the icosahedral phase is stabilized by the difficulty of rearrangement of atoms.