Reduplication forms (AA forms) occurred in Old Chinese, which functioned as content words. Some of them disappeared at a later stage, some others became the component of idioms, and still others became component of compounds which further became affix through the process of morphologization. This paper explores the process and mechanisms of the evolution of ABB pattern for adjectives. The reduplication affix in the pattern belongs to evaluative morphology in nature, having strong subjectivity. Studying on the origin and change of evaluative morphology should be a promising and especially needed aspect in the grammaticalization research of Chinese.