In a nationwide scale, we investigated the characteristics of disinfection facilities & operation practices of the water treatment plants. Disinfection requirement based on the SWTR (Surface Water Treatment Rule) of the U.S. (0.5-log and 1-log Giardia removal) was applied in estimating the compliance, considering only postdisinfection process in clearwells. All of the water treatment plants mainly used the chlorine as a disinfectant and 43 % of those adopted predisinfection. 55∼81% of the water treatment plants failed to satisfy CT value required for the removal of 0.5-log Giardia., depending upon the retention time based on design or operation. The plants showing the high non-compliance rate were largely than 50,000 ㎥/day. One of the main reasons was that the effective residence time was not achieved due to the lack of baffle wall in clearwells. Seventy plants, which failed to comply with the CT requirement for 2-log virus inactivation, were found.