Tooth root resorption due to an adjacent and/or involved tumor is considered to be one of the most important radiographic features for differentiating between benign and malignant tumors in the oral and maxillofacial region. Few studies have reported tooth resorption related to malignant tumors despite many reports of tooth resorption caused by benign tumors. This study was to investigate the frequency of root resorption and the relationship between the type of tumor extension and the root resorption in squamous cell carcinoma that is the most frequent type of oral cancer. Forty-five patients with squamous cell carcinoma involving the tooth root were examined retrospectively using radiographic findings. The type of tumor extension was divided into 2 patterns, the marginal type: tumor invaded from the alveolar margin and the apical type: tumor invaded from the root apex. The type of tooth resorption was divided into spiked type and knife-edge type according to the shape of the resorped root. In 45 cases showing tooth roots involved by the tumor, root resorption was recognized in 19 cases (42.2%). In 37 cases of the marginal type, root resorption was recognized in 15 cases (40.5%), all of which showed spiked type root resorption. All cases of the marginal type demonstrated gingival carcinoma. The remaining 8 cases were found to be apical type and all of these were maxillary cancer that filled more than two thirds of the maxillary sinus and extended into the oral region involving the tooth roots. Four of these 8 cases showed root resorption (50.0%), all of which were knife-edge type. In conclusion, tooth root resorption in squamous cell carcinoma may occur more frequently than previously considered. Only the spiked type was observed when root resorption occurred in the marginal type carcinoma. In the case of apical type maxillary carcinoma involving the oral and maxillary sinus region, knife-edge type root resorption may occur rather than the spike type.