Motions of the magnetotail boundary during substorms are investigated by the satellite data analysis. It is found that the magnetotail radius increases during 1--2 hours before the substorm expansion phase onset. Shortly after the onset time the tail radius begins to decrease rapidly and recovers to the presubstorm value generally within about 1 hour of the expansion phase onset. Time history of the increment ΔR in the tail radius resembles that of the lobe magnetic field magnitude. This confirms that both phenomena are manifestations of the increase in the total magnetic flux in the tail lobe. The average increment ΔR in the tail radius in the region -30RE?X?-70RE satisfies the inequality 0.5RE≪ΔR≪4RE, the probable value being 1--2 RE. This value accounts for 15--45% of the total flux increase in the tail lobe.