Abstract: In the light of Piercean Semiotics, by researching icons in Chinese characters and their evolution, it is to be investigated in the paper whether there is a continuum between image, diagram and metaphor and how the three types interact with each other. The study demonstrates that icons' iconicity can be pure or mixed; image, diagram and metaphor can all occur at the first level of signs. In the course of Chinese characters' evolution, some images developed into diagrams, and some images became metaphors through being used to represent other things similar to their original objects. A transitional icon type exists between diagram and metaphor, which is named metaphor schema. Metaphor schemata represent abstract concepts in the form of schemata. The conventionality of signs gradually increases from image-diagram- to metaphor, which embodies the symbolizing trend of icons. Image, diagram and metaphor constitute a continuum of icons.