The infection of virus will induce the accumulation of active oxygen species (AOS) in plants, which is a kind of spectrum resistance. But on the other side, active oxygen species can damage the plants, thus there are active oxygen species eliminating systems in plants, among which catalase (CAT) is one of the important AOS eliminating enzymes, whose function is to catalyze hydrogen peroxide (H₂O₂) to hydrogen oxide (H₂O) and molecular oxygen (O₂), the last substances are nontoxic to plants. Cucumber mosaic virus is one which does great harm to tobacco production. There are no reports about the metabolism of AOS in the interaction of tobacco and CMV in the world. To investigate the dynamic changes of catalase activity in the tobacco plants after the inoculation of CMV, several cultivators with different disease resistance were analyzed. The subject of the thesis is to find whether there is definite pertinence between catalase activity and disease resistance. The tobacco cultivators with different and stable disease resistance includes Shennong broad leaf, Tongzhou, Jintai 18, Qiannan 8, TI245, Tiebazi, K326, NC89, RG17, Lianghuang, G28 and G140. When the tobacco seedlings develop four to five leaves, cucumber mosaic virus was inoculated with the friction of diseased juice. The catalase activity was measured with different intervals after inoculation, and the state of disease were investigated twenty days after inoculation.