Sour cherry is a valuable fruit crop which is in demand on the European market. At present the interest to sour cherry fruits increase due to their high nutritional value and to sensory properties of a sour cherry. It promotes to the expansion of a cultivar assortment of a fruit crop and stimulates the acquisition of knowledge about the fruit's quality. The purpose of the research was to develop the mathematical model for the forecasting of a formation of fruit and stone weight in sour cherry cultivars irrespective of the environmental conditions. The research was conducted during 2007-2019 in the conditions of a Southern-Steppe zone of Ukraine. Theoretical, measuring and weighing, statistical. For the study, fruits from 10 varieties of cherries in the stage of consumer ripeness (100 pieces) were selected from 3-5 typical trees. The fruits were weighed and the weight of one fruit was determined. The average cherry fruit weight during the years of studying equalled 5.93 g, and the average stone weight equalled 0.39 g. Three sour cherry cultivars (Siyanets Turovtseva, Griot Melitopolskyi and Vstrecha) with the biggest average weight of a fruit from 7.48 t? 7.87 g were chosen. The sour cherry cultivars Siyanets Turovtsevoi, and Vstrecha were characterized by a big fruit weight under optimal variability of 16.68% and 18.82% respectively. The cultivar Shalunia had the lightest stone weight in sour cherry fruits. The optimal correlation between the stone weight and the cherry fruit pulp (4.8%) was determined in a Vstrecha. According to a two-way analysis of variance, the dominating effects of the environmental conditions on the formation of a fruit weight and on the formation of a stone weight in fruits of sour cherry have been established. The average monthly precipitation in May and the average monthly air humidity during a year, which received the 1st and the 2nd ranks, had decisive effects on the formation of fruit and stone weight in the fruits of a sour cherry.