It is often said that the English diphthongs underwent the most radical changes between Old English (OE) and Middle English (ME). The goal of this study is threefold: 1) to explore the new diphthongs in ME developed from full vocalization of the weak consonants [j, ɣ, w], 2) to analyze them within the framework of Optimality Theory (Prince & Smolensky 1993/2004, McCarthy & Prince 1993, 1995, among others) and 3) ultimately to show that an OT-theoretic analysis allows us to provide a unified account of the aspects of ME diphthongization through vocalization. This study shows that the markedness constraints such as *jcoda, *ɣ, and *wcoda, play a crucial role in triggering the vocalization of [j, ɣ, w]. These markedness constraints interact with other constraints to select the optimal outputs of ME diphthongs. The properties shared by [j, ɣ, w] in ME diphthongization can be captured by the same hierarchy of the constraints. Additionally, it is shown that the absence of short and overlong diphthongs in ME as well as vowel hiatus that arose from intervocalic diphthongization can be explained by the same hierarchy of the constraints.