Phase transitionwidely exists in nature, not only in ceramics, polymers andmetals, but also in biological systems. In material science peoplehave successively made use of martensitic transformations inmetallic systems for about three thousand years and the shapememory effect for a few decades. It seems from the behavior of theviruses and bacteria examined here that the earliest applicationof these phenomena dates back 3.5 billion years. But research inthis field is still at a beginning stage and of very limitedamount. This paper reviews several phase transition phenomena andmechanisms in biological systems, such as the tension of DNA, theinfection of the virus, especially the polymorphic transition ofbacterial flagellar filaments. We first give an introduction ofthe function and structure of the bacterial flagellar filament,and then present experiments about the phase transition ofbacterial flagellar filament, with attentions focused on themechanical force induced phase change and Ginzburg-Landau theorywhich is to describe the filament transition. Finally we reviewnew advances in the study of phase transition in biologicalsystems from both experimental and theoretical viewpoints.