This paper investigates iʔ5 ‘one’ in Shiposheng, a vernacular dialect spoken in the north of Guangdong Province. Though iʔ5 in Shiposheng and yi ‘one’ in Mandarin share similar usages, they behave differently when cooccurring with the demonstrative, i.e., iʔ5 in Shiposheng cannot follow the demonstrative as its Mandarin counterpart does. It is proposed that the numeral for ‘one’ in Shiposheng is grammaticalized to an indefinite article, while yi ‘one’ in Mandarin is still a numeral though it could be used as an indefinite article in some cases. The different behaviors of the Mandarin yi ‘one’ and the Shiposheng iʔ5 are due to their distinct grammatical status.