Objective: The aim of our study was to compare the value of computed tomography (CT) and 99mTc-methylene-diphosphonate (MDP) SPECT (single photon emission computed tomography)/CT fusion imaging in determining the extent of mandibular invasion by malignant tumor of the oral cavity. Methods: This study had local ethical committee approval, and all patients gave written informed consent. Fifty-three patients were revealed mandibular invasion by malignant tumor of the oral cavity underwent CT and SPECT/CT. The patients were divided into two groups: group A (invasion-periphery-type) and group B (invasion-center-type). Two radiologists assessed the CT images and two nuclear medicine physicians separately assessed the SPECT/CT images in consensus and without knowledge of the results of other imaging tests. The extent of bone involvement suggested with an imaging modality was compared with pathological findings in the surgical specimen. Results: With pathological findings as the standard of reference, Group A: The extent of mandibular invasion by malignant tumor under-went SPECT/CT was 1.02 ± 0.20 cm larger than that underwent pathological findings. And the extent of mandibular invasion underwent CT was 1.42 ± 0.35 cm smaller than that underwent pathological examination. There were significant difference among the three methods (P < 0.01). Group B: The extent of mandibular invasion by malignant tumor underwent SPECT/CT was 1.3 ± 0.39 cm larger than that underwent pathological examination. The extent of mandibular invasion underwent CT was 2.55 ± 1.44 cm smaller than that underwent pathological findings. There were significant difference among the three methods (P < 0.01). The extent of mandibular invasion underwent SPECT/CT was the extent which surgeon must excise to get clear margins. Conclusion: SPECT/CT fusion imaging has significant clinical value in determining the extent of mandibular invasion by malignant tumor of oral cavity. Objective: The aim of our study was to compare the value of computed tomography (CT) and 99mTc-methylene-diphosphonate (MDP) SPECT (single photon emission computed tomography) / CT fusion imaging in determining the extent of mandibular invasion by malignant tumor of The oral had had oral mandibular invasion by malignant tumor of the oral cavity underwent CT and SPECT / CT. The patients were divided into two groups: group A (invasion-periphery-type) and group B (invasion-center-type). Two radiologists assessed the CT images and two nuclear medicine physicians separately pre- the SPECT / CT images in consensus and without knowledge of the results of other imaging tests. The extent of bone involvement suggested with an imaging modality was compared with pathological findings in the surgical specimen. Results: With pathological findings as the standard of re ference, Group A: The extent of mandibular invasion by malignant tumor under-went SPECT / CT was 1.02 ± 0.20 cm larger than that underwent pathological findings. And the extent of mandibular invasion underwent CT was 1.42 ± 0.35 cm smaller than that underwent pathological examination There was significant difference among the three methods (P <0.01). Group B: The extent of mandibular invasion by malignant tumor underwent SPECT / CT was 1.3 ± 0.39 cm larger than that underwent pathological examination. The extent of mandibular invasion underwent CT was was 2.55 ± 1.44 cm smaller than that underwent pathological findings. There were significant differences among the three methods (P <0.01). The extent of mandibular invasion underwent SPECT / CT was the extent which surgeon must excise to get clear margins. Conclusion: SPECT / CT fusion imaging has significant clinical value in determining the extent of mandibular invasion by malignant tumor of oral cavity.