To develop new natural fungicides, six monoterpenes and two phenylpropenes were tested for their antifungal activity against eight plant pathogenic fungi. The results of the mycelial growth inhibition assay showed that trans-cinnamaldehyde was the most potent compound against the eight tested fungi with EC50 values ranging between 0.75 and 3.19 mg/l. This compound caused the higher mycelial growth inhibition than carbendazim. Furthermore, (-)-menthone exhibited strong antifungal activity against Alternaria solani (EC50 = 9.31 mg/l), Penicillium digitatum (EC50 = 16.14 mg/l), and Rhizoctonia solani (EC50 = 24.69 mg/l). Likewise, eugenol showed potent antifungal activity against P. digitatum, R. solani, Fusarium solani, and A. solani, whereas EC50 values were less than 30.0 mg/l. In a separate experiment, trans-cinnamaldehyde, p-cymene, eugenol, and (-)-menthone were evaluated for their inhibitory effects on pectin methyl esterase and cellulase. The tested compounds exhibited the pronounced inhibition of enzyme activities with trans-cinnamaldehyde being the most potent inhibitor for both enzymes.