Two field experiments were conducted at Wadi El-Natrun, El-Bahira Governorate, (latitude of 30.48⁰ N and longitude of 30.50⁰ E) during 2013/2014 and 2014/2015 seasons to study the effect of potassium and magnesium fertilizers on yield and chemical constituents of sugar beet (Beta vulgaris var. saccharifera, L.) grown in a sandy soil. The present work included twelve treatments, which were three potassium levels (50, 75 and 100 kg K₂SO₄/fed) added before canopy closer as potassium sulphate (48% K₂O), in combination with four levels of magnesium: (without magnesium, 20, 50 and 80 kg MgSO₄/fed in the form of magnesium sulphate (MgSO₄. 7 H₂O) containing 10% MgO, which was added after seed bed preparation and after plotting. The treatments were arranged in a strip plots design in three replicates. The vertical plots were occupied with the three levels of potassium and the horizontal plots were devoted to the four levels of magnesium. The results showed that fertilizing sugar beet with 75 kg and/or 100 kg K₂SO₄/fed produced significantly higher valuesof chlorophyll (b) and carotenoids, root fresh weight/plant, sucrose% and foliage contents of K and Mg, in both seasons. Applying of 75 kg K₂SO₄/fed significantly decreased the amount of Na, K, alph amino-N contents in roots, sugar lost to molasses% and increased quality index as well as extractable sugar, in both seasons. Meantime, chlorophyll (a), nitrogen and phosphorus contents in leaves, magnesium uptake in roots, root diameter and foliage fresh weight/plant, root/top ratio were insignificantly affected by potassium levels studied. Increasing magnesium fertilizer up to 80 kg MgSO₄/fed resulted in higher values of photosynthetic pigments, root and top fresh weights/plant, root/top ratio, foliage contents of N, K, P and Mg as well as magnesium uptake in roots. Moreover, it improved sucrose, extractable sugar and quality index percentages significantly, in both seasons. Meantime, potassium and sodium contents in beet roots were insignificantly influenced by the applied magnesium levels. The interaction between potassium and magnesium levels had a significant effects on leaf content of chlorophyll (a), root diameter, nitrogen content in leaves and root yield/fed, in both seasons as well as sugar yield/fed, in 1^st one. The combination of 75 kg K₂SO₄/fed + 80 kg MgSO₄/fed achieved the maximum root yield (27.88 and 27.07 t/fed) in the 1^st and 2^nd season, respectively and the highest sugar yields (4.51 t/fed) in 1^st one . It could be concluded that the application of K₂SO₄ at the rate of 75 kg and MgSO₄ at the rate of 80 kg is the suitable recommendation to maximize the productivity and quality of sugar beet grown in a sandy soils at Wady El-Natrun.