The allelopathic effect of leaf, root and bark extract of Guaiacum officinale on seed germination of Pennisetum glauccum (millet) were studied by applying three treatments (10, 25 and 50% extracts of each). Maximum effect on germination was observed in 50% leaf extract of Guaiacum officinale, in which only 10% seeds were germinated, compared to bark 70% and root 70.33% respectively. It was found that the seedlings grown in 50% leaf extract have the lowest dry weight of 0.016 g compared to seeds grown in bark and root extracts of Guaiacum officinale (0.06 and 0.68 grams respectively). The average root length was observed to decrease with the increasing concentration of root and leaves extract of the plant, the lowest root length was observed in root extract (0.59 cm) as compared to bark and leaves extract (3.43 and 1.33 cm respectively). The lowest shoot lengths were observed in root extract (1.96 cm) as compared to bark and leaves extracts of the plant (1.96 and 4.0 respectively).