This study was conducted to survey trihalomethane(THMs) levels in finished water of J water treatment plant and examine its reduction by powder activated carbon(PAC) treatment. Samples were collected weekly based and head-space technique was employed to determine THMs levels by G.C-ECD. THMs levels in finished waters were highest in August and showed close relationship with water temperature. All the samples satisfied the drinking water limit(100 ${mu}$g/l) for THMs. The individual formation rates of THMs were 64.8% for CHCl$_3$, 28.4% for CHCl$_2$BR, 6.5% for CHClBr$_2$ and 0.3% for CHBr$_3$, respectively and showed little monthly difference. The reduction efficiency of THMs formation by PAC treatment was 67% during July and August. Bromine substituents were more efficiently reduced than CHCl$_3$ by PAC.