This study was carried out to disclose the effect of cluster thinning at different stages as a method for crop adjusting on growth. yield and fruit quality of Ruby seedless grapevines. One-quarter of the cluster number on each vine was removed at different stages of cluster development. thus. reducing the number of clusters per vine from 32 to 24. Vines were randomly assigned a date of cluster thinning based on the following stages of cluster development pro-bloom (PB); full bloom (FB); berry set + one week after berry set (BS1); two weeks after berry set (BS2); three weeks after berry set (383) and four weeks after ben'y set (BS4) and unthinned vines (control), on which 32 clusters were retained through the experiment. The results showed that the quality of Ruby seedless grapes could be easily improved by the cluster thinning. The early thinning of clusters pre— blcom (PB) ensured the best vegetative growth parameters and the highest bud fertility. However. Vines thinned at one week after berry set (BS1) can be recommended as the best effective treatment. The slight decrease in the yield obtained from this treatment could be compensated by improving physical properties of berries and decreasing percentage of shot bern‘es per cluster . as well as achieving a highest percentage of TSS. better colouration and a lower percentage of acidity in the juice.