The p53 which is well known as tumor suppressor gene is located at 17p13. p53 is a sequence-specific DNA binding transcription factor that responds to certain cytotoxic stresses, such as DNA damage, by enhancing the transcription of genes that regulate cell-cycle progression as well as programmed cell death. The p63 gene that is located at 3q27-29, is recognized members of the p53 family, and responsible for the transcription of 6 isoforms. Three isoforms ($TAp63{alpha}$, $TAp63{eta}$, $TAp63{gamma}$) contain an N-terminal transactivation (TA) domain and can induce apoptosis. The other 3 isoforms (${Delta}Np63{alpha}$, ${Delta}Np63{eta}$, ${Delta}Np63{gamma}$) lack the TA domain and may function in a dominant-negative fashion by inhibiting the transactivation functions of p53 and TAp63 proteins, and thus act as oncoproteins. A number of studies have investigated the role of p63 in human squamous cell carcinomas from different organs. Only a few studies have examined ${Delta}Np63$ isoform in oral squamous cell carcinoma including normal epithelium. This study aimed to evaluate expression of ${Delta}Np63$ isoform in human oral squamous cell carcinoma tissue and normal mucosa. The 3 cases of well differenciated oral squamous cell carcinoma specimen including adjacent normal mucosa were examined, and immunohistochemical study with monoclonal antibody(4A4) and tumor cell apoptosis analysis with Transmission Electon Microscopy were studied. And, RT-PCR analysis was done for expression of ${Delta}Np63$ isoform. The results were as followed. 1. Normal gingiva showed the restricted p63 expression in basal cell layer. 2. Well differentiated squamous cell carcinoma showed mainly p63 expression in overall area of malignancy, especially in basal cell layer to adjacent stromal tissue. 3. Tumor cells around keratinized area with no p63 expression disclosed less micro-organelle in decreased size cytoplasm and severe chromatin margination with nuclear destruction that means apoptosis. 4. Comparison of mRNA expression of ${Delta}Np63$ isoform by RT-PCR showed variable expression of ${Delta}Np63$ isoform, but ${Delta}Np63{alpha}$ was most highly expressed in all 3 tumor specimen. From theses results, it should be suggested that ${Delta}Np63$ isoform expression in well differentiated squamous cell carcinoma was closely related to tumor oncogenesis, expecially overexpression of ${Delta}Np63{alpha}$ is a most important factor in tumor genesis of oral squamous cell carcinoma.