Caffeic and p-coumaric acid esters found in plants inhibited the growth of plant pathogenic fungi and bacteria. Many studies have been carried out to search for the antibacterial activities of the related compounds. p-Coumaramide isolated from Welsh onions indicated aglucosidase inhibition, and the related amide compound N-p-coumaroyloctopamine was induced from potato tuber disks treated with hyphal cell wall components of Phytophthora infestans. Ramanan et al. reported that the introduction of halogen onto the benzene ring of cinnamic acids enhanced the antimicrobial activity against Gram negative bacteria. In this paper, we describe the fungitoxic and phytotoxic activities of substituted cinnamate and cinnamamide derivatives. We synthesized substituted cinnamic acid derivatives (42 cinnamates and 45 cinnamamides), tested their fungitoxic activity against Pythium sp. and Corticium rolfsii by agar dilution method, and phytotoxic activity against radicle and hypocotyl growth of Brassica rapa var. amplexicaulis. Results showed methyl 4-chlorocinnamate (O-19) had the highest fungitoxic activity against C. rolfsii and plant growth inhibitory activity against Brassica rapa. Among cinnamamides, compound N-4 with butyl group showed 45% and 33% inhibition against C. rolfsii and Pythium sp. at 10 ppm respectively, while elongation of carbon chain over eight carbons decreased activity, and N-13 with sixteen carbon atoms had no activity. Most compounds inhibited radicles more than hypocotyls.