Thompson Seedless grapevines grown in clay soil were used in this work to determine the amount of potassium to be added per feddan and the proper time for its application.  Potassium was added as potassium sulphate (48% K₂O) at 0, 50, 75, 100 and 125 Kg  K₂O/feddan.   The assigned amount/treatment was divided into two or three parts applied in the first week of March, May and in August after harvesting.  The application of potassium significantly increased K content in the leaf petioles during bloom, veraison and maturation stages.  The highest values were in bloom stage than the other stages.  Potassium increased N percentage in leaf blades and starch in the canes.  Ca and Mg were decreased and P% was not affected.  TSS, TSS/acidity, cluster weight and yield/vine were significantly increased, while acidity was significantly lowered by increasing K₂O amount.  The most effective treatment was 125 Kg K₂O.