On 20 September 1999 UT (21 September in local time, LT) a large earthquake Mw=7.7 struck central Taiwan near the small town of Chi-Chi. The greatest plasma frequency in the ionosphere, foF21, observed by the Chung-Li ionosonde (25.0°N, 121.2°E) reveals three clear precursors at 1, 3, and 4 days prior to the earthquake. This paper examines the ionospheric total electron content (TEC) observed by a network of the global positioning system (GPS) receivers in Taiwan area. It is found that variations in foF2 and overhead TEC recorded at Chung-Li have a similar tendency. Combining the data of the network of 13 GPS receivers, time, and spatial variations of TEC prior to the Chi-Chi earthquake are examined. Results show that the equatorial anomaly crest moves equatorward and its TEC value significantly decreases 1, 3, and 4 days before the earthquake. A comparison between the disturbed and reference (previous 15-day median) days confirms that TEC decreases significantly around the epicenter in the afternoons of these days. Finally, possible mechanisms are proposed and discussed.