In order to investigation of allelopathic effects of medicinal plants on germination and seedling growth of different weeds, an experiment was done in Biotechnology Laboratory, Islamic Azad University-Mashhad Branch, and Iran. The experiment was conducted according to the completely randomized design (CRD) as factorial having two factors with four replications in 2011. Three different species of weeds including Peganum Harmala, Alyssum, Amaranthus Retroflexus and concentrations of shoot dry matter at five levels consisting of (0, 37, 75, 112 and 150 powder gram) of Mint, Rosemarinus, Lavandula and Achilla were applied. Results of data analysis showed that the effect of different levels of shoot dry matter concentration on germination percentage, plumule length, radicle length, seed vigor and seedling fresh weight was significant (p<0.01) and with increasing concentration, GP, PL, RL, SV and SFW were decreased considerably. Also, results revealed that Alyssum and Peganum were more sensitive to the inhibitory effects of dry matter concentration of medicinal plants in all the measured traits except for seed vigor index of Peganum.