A total of seventeen phytochemicals including seven alkaloids (piperine, strychnine, brucine, stachydrine, tetrandrine, frangchinoline and sinomenine), four phenols (paeonol, honokiol, magnolol and eugenol) and six anthraquinones (emodin, rhein, chrysorphanol, aloe-emodin, physcion and 1,8-dihydroxyanthraquinone) was examined for inhibitory activity of monoamine oxidase (MAO) A and B from rat brain mitochondrial. Among these compounds, piperine and paeonol were found to be inhibitory against MAO A in a dose-dependent manner with IC50 values of 49.3 and 54.6μM, respectively. Piperine, paeonol and emodin were shown to inhibit MAO B in a dose-dependent manner with the IC50 data of 91.3, 42.5 and 35.4μM, respectively. Lineweaver-Burk transformation of the inhibition data indicated that the inhibitory action of piperine onMAO A was of mixed type, and that of paeonol on the same type of the enzyme was of non-competitive type. For piperine, the Ki and KI were determined to be 35.8 and 25.7μM, respectively. For paeonol, the Ki was estimated to be 51.1μM. The inhibition of piperine and paeonol on MAO B was of competitive type with Ki values of 79.9 and 38.2μM, respectively. The inhibition of emodin on MAO B was of mixed type with the Ki and KI data of 15.1 and 22.9μM, respectively. The present investigation showed that the phytochemicals piperine, paeonol and emodin are potent MAO inhibitors whereas other compounds were inactive against any type of MAO at 100μM in the present assay.