The paper presents the results of chemical thinning of sour cherry flowers of the Šumadinka and the Čačanski rubin cultivars. The treatments were performed in the full flowering phase, using 0.0025% concentration of the LG-GERATS (ammonium thiosulphate). Following the application of the LG-GER-ATS, the number of set fruits was reduced by 9.8% and 8.0% for the Šumadinka and Čačanski rubin cultivars respectively, compared to trees receiving no treatment. The thinning of the flowers had an impact on the increase of the fruit mass by 19.1% and 10.9% in the Šumadinka and the Čačanski rubin cultivars respectively. Trees treated with LG-GER-ATS recorded a higher yield in both years of trial – the yield in Šumadinka cultivar was higher by 20.7% per tree, while the yield of the Čačanski rubin cultivar was higher by 17.1% per tree.