The spread pattern of a tumour and its extent in the mandible are important in the management of gingival cancer. Sixteen patients with gingival squamous cell carcinoma (SCC) involving the mandible in the molar region were included in this study. Resection specimens of the mandibular bone and adjacent cancer were histologically analysed to identify the type and characteristics of invasion and were compared with the radiological features. Our results showed that the actual width of invasion was underestimated to a greater extent than the actual depth of invasion. For horizontal aspects, four dentate cases had horizontal intramedullary spread underneath intact mucosa or cortical bone extended from the main foci of tumour that infiltrated through the occlusal surface. For vertical aspects, nerve invasion took place in only one of 16 specimens, while five cases showed downward infiltration beyond the inferior alveolar canal without nerve involvement, so that the pattern of tumour spread was mostly transmedullary rather than perineural in previously non-irradiated cases. These cases with deep infiltration showed the infiltrative type of invasion in the dentate mandible. And when the tumour was related with previous dental extractions or curettage, it tended to be more extensive than what was predicted from an imaging point of view. These pathological and clinical features affecting the tumour spread should be considered in the management of gingival SCC in the molar region.