Irritation fibroma is a reactive growth of fibrous connective tissue as response to local irritation such as ill-fitting immediate denture. It is most common on the buccal mucosa but only rarely on the labial mucosa. Diagnosis is based on clinical and histopathological examination. Information and education all about immediate denture is very important to prevent these lesions. The purpose of this article is to report an unusual case of irritation fibroma on the upper labial mucosa due to use of an ill-fitting immediate denture. A 55-year-old female complained of a lump on the labial mucosa since one half years ago. The lump was first noticed after 6 months using immediate denture. The size was like a corn kernel and painless but eventually became bigger. However, the patient sometimes feels sore and uncomfortable. She had a habit of playing ill-fitting denture, wearing it while sleeping and never controlling it to the dentist. On clinical examination, we found an exophytic lesion growing on the upper labial mucosa near the edge of her immediate denture. The diagnosis, confirmed by histopathological examination, was irritation fibroma caused by an ill-fitting denture. The pedunculated mass was excised for biopsy and histopathological examination was performed to confirm diagnose. The ill-fitting immediate dentures was withdrawn and replaced by one fitting denture. Irritation fibroma can be caused by wearing ill-fitting immediate dentures. Irritants can be avoided if the denture is controlled and worn according to the instructions. The dentists must provide education and information about the use, treatment, and follow-up of immediate dentures to avoid the occurrence of lesions due to the use of ill-fitting dentures.