It is common limiting technological problem and research focus of whole world brewing industry to improve the flavor stability with an aim to keep refreshing character of beer as long as possible to attract consumers in the shelf life of beer. A decrease of the bitterness intensity accompanied by a shift of the taste profile toward harsh and long lingering bitter after is well known phenomena observed during aging of beer. With the help of modern purification technologies, nuclear magnetic resonance spectroscopy and high resolution HPLC MS, the overseas researchers have took some breakthroughs in identifying the compounds responsible for harsh and lingering bitterness and other newly found bitter substances during brewing and storage process of beer as well as their possible transformation routs. In this paper, the highlights of the newly identified bitter compounds originated from hop a- acid were reviewed in detail, especially the formation mechanism, thresholds and elution of newly found tri- and tetra-cyclic molecules, namely main iso-α acids degraded compounds, during brewing and storage were introduced, in order to enlarge the knowledge of beer bitterness of beer brewing peers in China.