Aim To evaluate the antimicrobial effects of essential oils (EOs) from cassia, basil, geranium, lemongrass, cumin and thyme, as well as their major components, against Phytophthora parasitica var. nicotianae; to investigate morphological changes in hyphae and sporangia in response to treatment with cinnamaldehyde; and to further evaluate potential biocontrol capacities against tobacco black shank under greenhouse conditions. Methods and Results The results revealed that the extent of mycelial growth inhibition was primarily dependent on the composition and concentration of the EOs and the structure of individual compounds. Cinnamaldehyde had a significantly higher inhibitory effect on mycelial growth, formation of sporangia, and production and germination of zoospores in P.parasitica var. nicotianae in vitro, achieving complete inhibition of these phenotypes at 72, 36, 36 and 18mg l-1, respectively. Scanning electron microscopic observations revealed that cinnamaldehyde can cause considerable morphological degenerations of hyphae and sporangia such as cytoplasmic coagulation, shrivelled mycelia and sporangia aggregates and swelling and lysis of mycelia and sporangia walls. In vivo assays with cinnamaldehyde demonstrated that this compound afforded protective effect against tobacco black shank under greenhouse conditions in susceptible tobacco plants. Conclusions The results of in vitro and in vivo bioassays, together with SEM imaging of the microstructure of P.parasitica var. nicotianae supported the possibility of using cinnamaldehyde as a potent natural biofungicide in the greenhouse. Significance and Impact of the Study This study provides a theoretical basis for the potential use of cinnamaldehyde as commercial agents or lead compounds that can be exploited as commercial biofungicides in the protection of tobacco plants from P.parasitica var. nicotianae infection.