General methods for development and discovery of antitumor agents from plant kingdom are briefly described. The development, antitumor activity, mechanism of action, and clinical application of several naturally occurring antitumor agents, such as glyfoline, podophyllotoxin, vinblastine, taxol, camptothecin, and annonaceous acetogenins and their analogues are cited in this article as examples for the research in the field of natural antitumor agents.