In the course of grammar history, the initial endeavor to classify words only according to meaning, like early philosophers', was replaced by other trials to sort them on the basis of function and form. Thrax classified words into eight categories and his classification is based not only on meaning but also on function and form. Since Thrax, there have been much progress and many changes in English grammar. Otto Jespersen classified words into five categories and he tried to classify words only by their form. However, in some cases, there was a need to consider their meaning, so he worked out both three-rank theory of primary, secondary and tertiary, and junction and nexus theory. In this study, I claimed that Jespersen's word classes and theories are ideal way to search and find out a more perfect linguistic theory. He assumed just five lexical categories: substantives, adjectives, pronouns, verbs, and particles. We can find out this economy of classifying word classes in studies of generative grammarians such as Radford and Aarts. Better understanding Otto Jespersen's theories contributes to our effort for a better linguistic theory.