After-cooking darkening is a gray-black discoloration of the potato tuber, formed after cooking by the oxidation of an iron-chlorogenic acid complex. Cinnamic acid 4-hydroxylase (C4H) is a key enzyme involved in the biosynthesis of chlorogenic acid. The full-length c4h gene was cloned and sequenced from both genomic DNA and cDNA of Russet Burbank tuber tissue by PCR and 5' and 3' RACE. The gene expression levels of c4h were examined by Northern hybridization, relative quantitative RT-PCR and real time quantitative RT-PCR in potato cultivars and wide selection of diploid clones varying in susceptibility to after-cooking darkening. Results suggest that there is a relationship between the levels of c4h gene expression and the degree of after-cooking darkening in potato tubers. The inhibition of C4H gene expression and over expression of C4H expression were also examined. The successful inhibition of the gene expression will lead to the reduced biosynthesis of chlorogenic acid, reducing the susceptibility of after-cooking darkening. The successful overexpression of the C4H gene will lead to the increase in the chlorogenic acid in plant tissues, gaining the resistance to diseases. In addition, due to the natural antioxidant activity of chlorogenic acid, overexpression of C4H gene will lead to its over production in plant tissues, such as potato tubers.