Astudy was conducted in 2002and 2003to determine the optimal degree of cluster thinning for crimson seedless grapevines. The vines were grown in a private vineyard at Wadi El Natron, El-Behera Governorate. The vines were 5-year-old. The treatments were: (1) control (the number of clusters per vine was adjusted to 40). (2) The number of clusters per vine was fixed at 35. (3) Number of clusters per vine was fixed at 30. (4) The number of clusters per vine was fixed at 25. The results indicated that, all thinning treatments exerted a substantial increase in average bunch weight, bunch dimensions berry weight and size and berry dimensions. The higher intensity of cluster thinning, the greater was the increase in these parameters. Ripening was hastened in thinning treatments resulting in the highest percentage of TSS, TSS/ Acid ratio, anthocyanine content of berry skin and the lowest acidity of berry juice. The results concerning the effect of treatments on storability and fruit quality during 50 days of cold storage at 0C, RH. 90-95 % cleared out that 25 cluster / vine gave the highest percentage of cluster fresh weight loss and TSS percentage and the lowest values of berry firmness and total acidity. It was concluded that the decrease obtained in the yield of vines thinned to 30 clusters/vine could be compensated by the higher quality and earlier ripening of clusters which undoubtedly ensure considerably higher prices especially in the foreign markets.