An earthquake of M_D = 5.7 occurred at Taipingshan, Ilan, Taiwan on May 10, 1983. A low seismicity zone appeared before the occurrence of the mainshock in the aftershock area. The epicentral pattern of aftershocks showed a significant correlation with an unexposed fault zone. The monthly frequency of events displayed abnormal seismicity migrated somewhat from the outside area to the source area. About one and a half years before the major event. The duration ratio (DR) of the total coda waves duration time from a distant station compared to that at a closer station is also studied as an earthquake precursor. The DR values immediately decreased after the mainshock. An overestimation of total coda waves duration times for the close-in station might be due to a mixture of seismograms generated from the events which occurred in a mixture of seismograms generated from the events which occurred in a small time interval and might result in this decrease. One hour after the mainshock, the DR values raised. This is interpreted to be a result of a large fracture density caused by the rapid increase of the number of M_D > 3 events immediately after the mainshock.