The magnetic island width during long-pulse, high- discharges was evaluated by measuring oscillations of electron cyclotron emission using a heterodyne radiometer. Because of its high spatial resolution of about 2 cm, evolution of the magnetic island structure can be measured in detail. The island width increases with the amplitude of the magnetic fluctuations. The saturated island width was about 7 cm when the m/n = 3/2 mode appeared. The resistive modes which limit the sustainable have typical mode numbers of m/n = 3/2 and 2/1. The relationship between the observed magnetic island and the neoclassical tearing mode is discussed.