Aim. Despite the successful development of endovenous laser obliteration (EVLO), the choice of energy parameters is often empirical. To select the optimal parameters of laser energy flow density necessary for complete denaturation of venous wall proteins. Material and methods. The ex vivo models of EVLO were based on the segments of the great saphenous veins removed from 9 patients presenting with primary varicosis. A total of 35 venous fragments were used for modeling and 6 ones for the hydrothermal experiment. The laser employed in the present study emitted the wavelengths of 1470 nm in the quasi-continuous regime (pulse duration 990 ms, pause length 10 ms) with the radiation power between 2 and 8 W. Laser radiation was delivered into the vein through a bare quartz fiber lightguide 600 mcm in diameter. Traction was performed either manually (1 mm/s) or instrumentally in an automatic regime with a traction apparatus operated at a preset rate of 0.5 or 0.7 mm/s. The linear energy density was 49.5-99 J/cm. Differential scanning calorimetry (DSC) was used to measure thermal stability of the samples of healthy and varicose veins altered by laser irradiation. Specifically, the minimal temperature required for complete denaturation of venous wall collagen was determined. Conclusion. It is concluded that denaturation of venous wall collagen is an integral index for the experimental choice of the energy required for reliable obliteration of the vein. DSC is an effective method that can be used to estimate the completeness of collagen denaturation and therefore the adequacy of laser radiation impact in the EVLO model. In order to achieve complete denaturation of collagen in the venous wall during EVLO, the temperature of adventitia should not be below 90 °C. The optimal option for bare fiber lightguides would be to use a 1470 nm laser with the energy flow density of at least 80 J/cm.