On 21 September 1999 (Taiwan local time), a major earthquake measuring M7.3 occurred near the town of Chi-Chi in central Taiwan. After the Chi-Chi earthquake, geomagnetic data recorded by a network of 8 stations equipped with continuous recording systems was analyzed. The results revealed that the total geomagnetic intensity of the Liyutan station, about 8 kilometers from the northern end of the Chelungpu fault (considered to be related to the earthquake), fluctuated significantly for more than a month prior to the earthquake. The fluctuation features continued and then stopped after the Chia-Yi earthquake (M6.2) occurred near the southern end of the Chelungpu fault on 22 October 1999. The variation of intensity reached 200 nTs. Geomagnetic fluctuations were also found at the Tsengwen station, located about 42 kilometers from the southern end of the Chelungpu fault and 30 kilometers from the Chia-Yi earthquake. These geomagnetic disruptions with highly anomalous amplitudes associated with the Chi-Chi and Chia-Yi earthquakes appear to have been the result of the accumulation and release of crustal stress that led to the subsequently severe surface rupture at the time of the earthquakes.