To study the antioxidant constituents from the stems of Dendrobium nobile, and to discuss their structure-activity relationship. Compounds were isolated from a 60% ethanolic extract by various chromatographic techniques and were identified by spectral analysis. The antioxidant activities of compounds were evaluated by DPPH free radical scavenging assay. Five phenanthrenes and four lignans were obtained from the active fractions of D. nobile. Their structures were identified as fimbriatone (1), confusarin (2), flavanthrinin (3), 2,5-dihydroxy-4,9-dimethoxyphenanthrene (4), 3,7-dihydroxy-2,4-dimethoxyphenanthrene (5), syringaresinol (6), pinoresinol (7), medioresinol (8) and lirioresinol-A (9), respectively. Compounds 2 and 6 exhibited more potent DPPH scavenging activities than vitamin C. All the above compounds were reported from this plant for the first time, and compounds 3, 4 and 9 were reported for the first time from the genus of Dendrobium. For all phenanthrenes and lignans, an electron-donating methoxyl group in the ortho position to the phenolic hydroxyl group exhibits enhanced antioxidant activities.