This study was carried out during the two successive seasons 2022 and 2023 on 12 years old" King Roby" grapes grown at Assiut Governorate, Egypt, to study the effect of spraying Kaolin  at 2&3% and Potassium silicate at 4&5% as preharvest on yield, berry quality, storability and control of postharvest diseases of grapes. All vines were sprayed three times at growth start (last March), just after berry setting (the first of May) and at one month later. Treated clusters were stored at (1±1 0C) and 90-95 % relative humidity for 4 weeks. The results showed that all treatments significantly improved the quantity and quality at harvest of “King Roby" grapes. potassium silicate treatment at 5% increased yield kg/vine, cluster weight (g), cluster length (cm), berry weight (g), TSS%, anthocyanin content (mg/100gFW), leaf area(cm2) and decreased number of shot berries, while 3% kaolin reduced number of sunburn. Throughout cold storage at (1±1 0C) and 90-95 %RH for 4 weeks, 2% Kaolin treatment improved TSS% and decreased weight loss%, shattering% , decay% and disease incidence by Botrytis Cinerea and other mold fungi, whereas 3% kaolin improved TSS/acid ratio and vitamin C (mg/ 100 ml juice). Potassium Silicate at 5% increased anthocyanin content (mg/100gFW) up to three weeks of cold storage and then decreased after that. It is recommended to spray  potassium silicate at 5% in grape farms to obtain a high yield and high quality. However, it is preferable to spray with kaolin at 2% to prolong the storage period, maintaining the quality of the clusters and protecting against fungal molds especially gray mold Botrytis Cinerea.