In May 4, 2008, a strange extreme long wave suddenly came from a relatively calm sea and swept dozens of people off the breakwater at a coastal site in the west coast of Korea, Boryeong (Fig.1). The sea state was calm with significant wave height less than 20 cm for most of the wave observation stations in the area and wind speed is also negligibly small. The tsunami-like abnormal long waves are generated by atmospheric pressure jump moving together with the waves over the Yellow Sea. Similar long wave occurred at the west coast of Korea in 2007 causing casualties of 4 at Yeoung-Kwang at the estuary, in which case the significant signals were detected from various stations along the west coast of Korea. However, significant long waves causing casual-ties occurred at a very limited area of Boryeong, The long wave generation and amplification mechanisms of the Boryeong abnormal waves synthesized from the weather radar image analysis, atmospheric pressure data analysis and the information reported by the local witnesses and analysis of the CCTV video image of the site are introduced and numerical experiments with long wave model are carried out for the Yellow Sea together with wave transformation near the local site to see the generation of long waves with movement of atmospheric jump and possible amplification of the long waves with topography and interaction with current. Feasibility of the prediction of such abnormal long waves will be also discussed.