The effects of geomagnetic storm on ionospheric total electron content (TEC) have been investigated by using the Global Positioning System (GPS) data during two observations on January 10 and May 15, 1997, respectively. It is found that after the onset of sudden storm commencement (SSC), the equatorial anomaly crests move poleward, and the daytime TEC is significantly reduced one day after SSC. Some possible mechanisms are proposed to explain the above phenomena. (C) 1999 COSPAR. Published by Elsevier Science Ltd.