Partial defoliation in the grape zone is one of the most common ampelotechnical procedures used in grape production in order to improve the microclimate of the vine and increase the quality of grapes. The partial defoliation procedure carried out at the beginning of the grape season has so far been common for continental Croatia due to colder and wetter climatic conditions, but due to the increase in air temperature in recent years it is necessary to review the different dates of conducting partial defoliation in the mentioned area. Earlier timings of partial defoliation are expected to affect the decrease in yield and the increase in the concentration of sugar and polyphenol compounds in the grapes of the 'Merlot' variety. The aim of the work is to determine the influence of partial defoliation on yield, the basic chemical composition and the concentration of individual polyphenol compounds in the grape of the 'Merlot' variety. The research was conducted at VVP Jazbina, on the Merlot variety, 2019. The partial defoliation procedure was performed in four timings, where four basal leaves were removed from each treatment, while the fifth treatment was controls. The timings of partial defoliation were: before flowering, immediately after flowering, when the grapes were pea-size and at the begining of veraison. The experiment was set up on Randomized Complete Block Design ─ RCBD in three repetitions. The experimental block consists of three adjacent vines, which means that each treatment is represented with nine vines. In the vineyard, at a time of full maturity, each vine was picked separately, and the average mass of bunches and yield per vine was determined. From each repetition, a sample of the must was taken from which the sugar content and total acidity was determined according to O.I.V (2001). A sample of 100 berries was also taken from each repetition, which separated the grape skin to determine individual polyphenols using the method of liquid chromatography.