Previous studies of Chinese diminutives usually fail to connect language descriptions to linguistic theories, and fall into a dichotomy between synchrony and diachrony. In view of these, this study, based on the diminutives in Yuebei Tuhua (YBTH), targets for the gradualness of diachronic change in terms of partially ordered constraints (POC) under the framework of optimality theory (OT). It first reviews how YBTH diminutives are formed, and how they are analyzed under the OT framework, based on Cheng (2006) and Chung and Cheng (2007). Then, POC is applied to these YBTH diminutives, with special attention given to two issues, noncrucial ranking and debuccalization. This study shows that POC works well to capture not only the motivations in diachronic change, but also the optionality in language variation.