the Search was conducted in General Commission for Scientific Agricultural Research in the Research Station Tartab / Research Center Qamishli / so as to determine the best substrate for the growth and production of Oyster Mushroom "Pleurotus ostreatus" To study the effect of adding different levels of bread yeast " Saccharomyces cerevisiae " for substrates used in the quantity and quality of production of the Oyster Mushroom.