Abstract: This paper introduces the methodology of preparation of samples medium heavy soils (20 – 45% wt.) for grain size analysis using laser diffraction. We focused on three different methods of preparation of samples that have been removed from the basin of the river Nitra. These methods are different in the amount used in the soil, by applying chemicals, distilled water and ultrasound energy level on the soil sample. Laser diffraction method provides a rapid solution to determine the soil particle size distribution especially in the analysis of large particles of soil samples. The advantage of laser diffraction is a short analysis of the samples, but this method does not introduce methodology of preparation of soil samples for analysis. The paper aimed to comparison of the results using a different method of preparation of samples soils with pipette method. The percentage distribution of grain size fractions were performed on the device analyzer ANALYSETTE 22 Micro Tec plus dispersing wet. Results of the particle size distribution, which resulting from the tree different methods of preparation of samples were compared with the standard pipette method.