One of the stylistic features in Samuel Richardson's epistolary novels is an abundant variety of negative expressions. Among them is included the addition of negative affixes such as dis-, in-, un- and less, by which new words are produced to convey new meanings and ideas. The present paper aims at exploring Richardson's use of negative prefixes. Our focus here is put on the analysis of "un-words." Types and frequencies of the un-words examined through the corpus of Richardson's three works(i.e. Pamela, Clarissa and Sir Charles Grandison) are shown in four tables. Referring to the word lists in the tables, we discuss how and why Richardson may have tried to create new word forms.