The objectives of the present study was to evaluate the influence of the carbonized layer at the fiber end face on the endovascular coagulation in varicose veins with a wavelength of 1885 nm measured when venous lumen is filled either with blood or the saline solution. Material and methods. Two groups of experiments on endovascular laser coagulation at a wavelength of 1885 nm were carried out on isolated veins. The venous lumen was filled either with the saline solution (1—3 series) or with the blood (4—6 series), respectively. The macro- and microscopic changes (×40) in the veins after coagulation were assessed. The X-ray fluorescence technique was used to determine the carbon concentration at the fiber end face. Results. In the first series of the experiments involving the application of the laser with an output power of 2.8 W, the pullback velocity of 0.6 mm/s, and the linear energy density of 46 J/cm the shrinkage of the vein was estimated at 18±3%. The concentration of carbon at the fiber end face was equal to 15±0.6%. The histological analysis has demonstrated the difference between inside and outside muscle layers. In the 4th series with the same parameters of coagulation the vein shrinkage was 45±3%. The carbon concentration was equivalent to 60±1,2%. Simultaneously, the damage to all layers of the venous wall was evaluated by the histological study. In both experimental series, the direct correlation between the increase of the laser output power and the decrease of the pullback velocity as the veins shrank. Conclusion. An important element responsible for the damage to the venous wall, is the formation of the carbonized layer at the fiber end face containing up to 60% of carbon which grows up in response to exposure of blood in the vein lumen to radiation with a wavelength of 1.885 mcm.