The morphology and characteristics of melanosomes are important features for differentiating melanocyte-derived melanotic lesions such as benign and malignant melanoma. There are two types of melanosome; eumelanin and pheomelanin. In this study, we attempted to elucidate the characteristics of melanosomes in malignant melanomas (melanotic type and amelanotic type) by ultrastructural analysis. In the melanotic type malignant melanoma in situ, pleomorphic and ellipsoid abnormal melanosomes, so-called eumelanin melanosomes, were detected, together with an increase in alkali elution rate. In the melanotic type invasive malignant melanoma, irregular ellipsoid and spheroid melanosomes were found either as discrete bodies or compound melanosomes, with further increase in alkali elution. In the amelanotic type malignant melanoma in situ, there were no melanosomes in atypical melanocytes and keratinocytes. In the amelanotic type invasive malignant melanoma, a few round melanosomes with low density matrix, so-called pheomelanin melanosomes, distributed in the cytoplasm. Our findings suggest that abnormal melanosome morphology and characteristics are predictive differential markers for the melanotic type or amelanotic type of oral malignant melanomas.