Oral squamous cell carcinoma (OSCC) develops through multistep process, that is, from normal mucosa to hyperplastic area and progressed to dysplasia, carcinoma in situ, and finally to invasive carcinoma. The purpose of this study is to investigate the histological types of the transitional area from normal oral mucosa to invasive carcinoma for the baseline data to search intermediate end point markers for early detection of OSCC. For this purpose, we reviewed the 85 patients who were diagnosed as OSCC in the Department of Oral Pathology, Yonsei University College of Dentistry, from 2002 to 2008. We classified these histopathologic findings by light-microscopy, according to the histologic pattern of transitional areas. As results, stepwise transformation from normal oral mucosa, to dysplasia and to OSCC was shown in 47 patients. Intermittent lesions were seen in 16 patients, in which normal oral mucosa, dysplasia, and OSCC were alternately arranged. Twenty two patients showed abruptly transformed to OSCC from normal oral mucosa. These preliminary data will be used for searching biomarkers for early detection of OSCC.