A cellular slime mold, Polysphondylium pallidum was isolated from the forest floor of Mountain Muhak. The effect of 11 selected monoterpenoids on the growth of P. pallidum was studied. We tested four different concentrations (1, 0.1, 0.01, and 0.001 $mu$g/$mu$l) for each compound by using a disk volatilization technique. Each compound was treated after germination of spores of P. pallidum. The growth of P. pallidum was inhibited by the treatment of the monoterpenoids at all concentrations tested. The microscopic analysis further supported these results. Most of the inhibitory effects of the compounds were represented by changes in the shapes of the fruiting bodies, such as very short sorophores, smaller sized sori and sori without spores. Especially, the monotepenoids changed the shape of whorls of side branches. These results suggested that selected monoterpenoids inhibit the growth of P. pallidum.