An apparatus and method for automatically preparing a mixture of particles of two or more substances such as but not limited to raw tobacco and raw medical cannabis flowers in one or more receptacle where the mixture proportion is predefined, is disclosed. The apparatus of the invention can be used for mixing any types of herbal substances. Moreover, the apparatus of the invention can be used for mixing one or more kinds of the same substance, for example mixing two or more kinds of marijuana strains. The apparatus includes one or more compartments for inserting each of the substances separately. In some variations of the present invention the size of at least one of the compartments is smaller for predefined smaller quantities of raw herbal material for example, a single marijuana flower.