Astudy was conducted in 2002and 2003to disclose the effect of berry thinning on yield, physical and chemical characteristics of clusters and berries of Crimson Seedless grapevines. The vines were 5-year-old grown in a private vineyard. The treatments were applied as follows: 8, 9, 10 shoulders per cluster and control. The results showed that the quality of Crimson Seedless grapes could be easily improved by berry thinning. However, vines thinned with leaving eight shoulders per cluster can be recommended as the best effective treatment. The decrease in the yield obtained from this treatment could be compensated by improving physical properties of berries as well as achieving a higher percentage of TSS, better colouration and a lower percentage of acidity in the juice.