The present work was done at the Experimental Farm of Faculty of Agriculture, ZagazigUniversity (Al-Khattara), Sharkia Governorate, Egypt during two consecutive seasons of 2012-2013 and 2013-2014. Different concentrations (0, 100, 200 or 300 mg/l) of L-phenylalanine or L-tryptophan each alone were foliar applied to milk thistle (Silybum marianum L.) plants to assess their effects on growth, productivity and chemical constituents. Obtained results demonstrated that the maximum values of plant growth characters (plant height, number of main branches, number of leaves /plant and herb fresh weight), yield components (number of fruits/plant, fruit weight, weight of 1000 seeds, seed weight/plant and seed yield/faddan) and chemical constituents (total nitrogen, phosphorus, potassium and total carbohydrates percentages in leaves as well as total phenolic compounds content of seeds) were recorded by spraying the plants with the highest concentration (300 mg/l) of each compound alone.