Both Cinnamomum verum J.S. Presl. and Cinnamomum cassia Blume are collectively called Cortex Cinnamonmi for their medicinal cinnamon bark. Cinnamomum verum is more popular elsewhere in the world, whereas C. cassia is a well known traditional Chinese medicine. An analysis of hydro-distilled Chinese cinnamon oil and pure cinnamaldehyde by gas chromatography/mass spectrometry revealed that cinnamaldehyde is the major component comprising 85% in the essential oil and the purity of cinnamaldehyde in use is high (> 98%). Both oil and pure cinnamaldehyde of C. cassia were equally effective in inhibiting the growth of various isolates of bacteria including Gram-positive (1 isolate, Staphylococcus aureus), and Gram-negative (7 isolates, E. coli, Enterobacter aerogenes, Proteus vulgaris, Pseudomonas aeruginosa, Vibrio cholerae, Vibrio parahaemolyticus and Samonella typhymurium), and fungi including yeasts (four species of Candida, C. albicans, C. tropicalis, C. glabrata, and C. krusei), filamentous molds (4 isolates, three Aspergillus spp. and one Fusarium sp.) and dermatophytes (three isolates, Microsporum gypseum, Trichophyton rubrum and T. mentagraphytes). Their minimum inhibition concentrations (MIC) as determined by agar dilution method varied only slightly. The MICs of both oil and cinnamaldehyde for bacteria ranged from 75 microg/ml to 600 microg/ml, for yeasts from 100 microg/ml to 450 microg/ml, for filamentous fungi from 75 microg/ml to 150 microg/ml, and for dermatophytes from 18.8 microg/ml to 37.5 microg/ml. The antimicrobial effectiveness of C. cassia oil and its major constituent is comparable and almost equivalent, which suggests that the broad-spectrum antibiotic activities of C. cassia oil are due to cinnamaldehyde. The relationship between structure and function of the main components of cinnamon oil is also discussed.