This dissertation attempts to analyze the development of used to through four historical corpora (the Penn-Helsinki Parsed Corpus of Middle English 2; the Penn-Helsinki Parsed Corpus of Early Modern English; A Representative Corpus of Historical English Registers; and the Corpus of Late Modern English Texts, Extended Version). This study first focuses on the frequency of words in order to find any significant increase in the trend of used to. The frequency of used to with inanimate subjects and stative verbs is also searched for so as to provide evidence for grammaticalization, which other studies of semi-modal have shown. The aspects of used to in negation and with personal subjects are also analyzed. However, because the results reveal that used to is a low frequency semi-modal, qualitative discussion about its grammaticalization is in need. In addition, the inconsistent system of negative forms of used to is discussed as well