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A decrease in Q values within the hanging wall of the ruptured Chelungpu fault two years following the 1999 Chi-Chi earthquake was revealed by Q tomography images and an analysis of single-path Q. The synthetic and sensitivity tests of the Q determination were carried out accordingly to justify the temporal variation. A Q value within the hanging wall above the hypocenter was determined to be 157 ± 18 two years following the Chi-Chi earthquake, which is significantly lower than the Q tomography values of 238 ± 17 and 289 ± 13 prior to and two years after the main shock, respectively. Similar values using a signal-path Q analysis from events within the ruptured fault zone to stations along the fault were obtained. The corresponding Q values were 247 ± 85 prior to the Chi-Chi earthquake. After the earthquake we obtained Q values of 158 ± 75 and 318 ± 80 for 2 years following and 2 years after the earthquake, respectively. Considering the two independent methods in determination of Q, the reduction in Q by 89 two years following the Chi-Chi earthquake in both methods is significant. Along with 1 % V reduction revealed by the analysis of repeating earthquakes our studies suggested possible reduction both in V and Q values within the fault zone after the Chi-Chi earthquake. Temporal changes in Q after the Chi-Chi earthquake imply variations of pore-fluid saturation in the ruptured fault zone. The reduction in Q two years following the Chi-Chi earthquake indicates high pore-fluid saturation within the fractured fault zone due to the postseismic fluid redistribution. [ABSTRACT FROM AUTHOR]