l-Ascorbic acid is an important antioxidant in both animals and plants. Monodehydroascorbate reductase is an enzyme involved in regulating the l-ascorbic acid level in plants. Here, we report the cloning of the reductase gene from non-heading Chinese cabbage (Brassica campestris ssp. chinensis Makino) and negative regulation of l-ascorbic acid and dehydroascorbate levels in transgenic tobacco plants expressing the gene under the control of the 35S promoter. The transgenic plants had reduced growth. The transgenic plants had changed the expression of the genes in the d-mannose/l-galactose pathway, degradation and recycling pathway and the activity of enzymes involved in degradation (ascorbate oxidase and ascorbate peroxidase) or recycling of ascorbic acid (dehydroascorbate reductase and glutathione reductase).