  We study the nu=1/3 quantum Hall state in presence of the random disorder. We
calculate the topologically invariant Chern number, which is the only quantity
known at present to unambiguously distinguish between insulating and current
carrying states in an interacting system. The mobility gap can be determined
numerically this way, which is found to agree with experimental value
semiquantitatively. As the disorder strength increases towards a critical
value, both the mobility gap and plateau width narrow continuously and
ultimately collapse leading to an insulating phase.
