The plant disease produced by Sclerotinia sclerotiorum is a big problem in agriculture. This paper tested the effects of cinnamon oil and its components on the growth of S. sclerotiorum. The cinnamon oil was first separated into carbonyl and noncarbonyl parts by treatment with sodium bisulfite. Then two main cinnamaldehyde derivatives, that is 2â²-methoxycinnamaldehyde and coniferyl aldehyde, were further separated by column chromatography from the carbonyl parts. Cinnamon oil demonstrated a significant antifungal effect against S. sclerotiorum with a minimum inhibitory concentration (MIC) of 256 Î¼g/mL in agar and 64 Î¼g/mL air, respectively. trans-Cinnamaldehyde exhibited the highest antifungal activity among all the three cinnamaldehydes tested. In addition, thymol and carvacrol had an additive effect with trans-cinnamaldehyde in preventing the mycelial growth of S. sclerotiorum.