The purpose of this paper is to analyze the semantic extension of English -er nominals from the perspective of cognitive linguistics. The prototypical sense of the English nominating suffix -er is a professional human agent. First of all, we discuss the basic sense of -er nominals. We further investigate the metaphorical and metonimic meaning extension of the -er nominals from professional human agents to the domain of instrument, object, event, location, and animals․plants. Demonstrating that the English -er nominals are polysemous word whose meanings are conceptually and coherently related through metaphorical and metonimic links, we propose the semantic network of the polysemous -er nominals based on Langacker's(1991) semantic network model. The main point of this proposal is that the various meanings associated with polysemous words such as English -er nominals are conceptually related to each other in the networks of meaning, each with a central prototype and peripheral senses linked to the prototype.