This paper endeavours to analyze the registers and the development of Peh-ōe-jī (Romanized colloquial) using concepts from language planning. In order to challenge some misunderstandings about Peh-ōe-jī. The paper attempts to highlight aspects of the language's secularization, localization and modernization. Language modernization is defined as including graphization, standardization and modernization. Languge localization is defined mainly according to identity and actual use. More than a tool for communication, language is also a symbol of identity. Furthermore, language changes according to social change. Peh-ōe-jī has achieved both modernization and localization, and has become an important part of Taiwan's linguistic landscape. However, it lacks official recongnition, hence has no institutional support. The paper ends with a suggestion to promote Peh-ōe-jī from a language planning perspective, which consists of status, corpus, acquisition and marketing planning.