Abstract: More and more data of plant chitinases have been accumulated. Some plant chitinases not only have the activity of degrading chitin but also have others activities. Typical chitinases have a N_terminal signal region, a catalytic domain and a C-terminal region, but some chitinases have one or more chitin_binding domains. Every domain has its own function. The classification system of plant chitinases has been revised several times and the present system makes up of 4 classes containing 9 families. There were transpositions during the evolution of plant chitinases, which has been universally accepted, but the views on the order of their evolution are different. Understanding the relation between chitinases and some others proteins can help to understand the evolution and origin of chitinases. Chitinase genes have broad application in genetic engineering of plant resistance to fungal diseases due to the special anti_fungal character of chitinases.