Four phenylpropanoid antialgal compounds (1-4) were successfully isolated from Gracilaria lemaneiformis through a combination of solvent soak, liquid-liquid extraction, silica gel column chromatography and repeated preparative thin-layer chromatography. On the basis of the spectral data, the compounds were identified as benzene-1,2-propanoic acid (1), gossonorol (2), 7,10-epoxy-ar-bisabol-11-ol (3), and p-hydroxyphenylethanol (4). These 4 compounds were isolated from marine macroalgae for the first time. Further, a quantitative relationship between the inhibition of algal growth and the concentration of each phenylpropanoid antialgal compound was determined and important parameters for future practical HAB control, e.g. EC_{50-96 h}, were also obtained. The results indicated that isolated compounds 1-4 possessed selective antialgal activity against the growth of several red tide microalgae (Amphidinium carterae, Heterosigma akashiwo, Karenia mikimitoi, Phaeocystis globosa, Prorocentrum donghaiense, and Skeletonema costatum). Their antialgal activity against the tested red tide microalgae has not been previously reported. Among them, benzene-1,2-propanoic acid (1), gossonorol (2) and p-hydroxyphenylethanol (4) showed more extensive antialgal activities against the tested red tide microalgae. Furthermore, benzene-1,2-propanoic acid (1) against A. carterae, H. akashiwo and P. globosa; gossonorol (2) and p-hydroxyphenylethanol (4) against H. akashiwo and P. globosa; and 7,10-epoxy-ar-bisabol-11-ol (3) against H. akashiwo and K. mikimitoi possessed better application potential than potassium dichromate as a characteristic antialgal agent against these red tide microalgae.