Abstract: Oral lenkoplakia and lichen planus are precancerous lesions, in which malignant hange may occur For the early diagnosis of malignant change, basing upon the experience of xperimental study we did before, several non-traumatic examination methods were used in linical cases. Results showed that the accurate rates of Toluidine blue staining and ematoporphyrin derivative （Hpd） following local application is 90 percent. Transmission lectron microscopic study of cytology is one new detection method indicating that cellular icrostructure is distinctly different between malignant tissue and normal mucous menbrane or eukoplakia with simple hyperplasia.