During last years, ultrasound duplex angioscanning (USD) gives opportunity to get the most complete information respecting the venous pathology and is used as a method of navigation during conducting of endovascular manipulations. Purpose of research — to study the ultrasound changes in the thrombosed vein before and after the influence of endovascular high-frequency electric welding and to evaluate the effectiveness of the method in the complex surgical service of acute ascending thrombophlebitis of the GSV. During the period from 2015 to 2017 in the surgical departments of the city clinical hospital №8 in Kyiv there were 38 patients with acute ascending thrombophlebitis of the great saphenous vein (AATGSV), who were treated with endovascular electric welding (EVEW). The age ranged from 19 to 78 years (average age 51.28±2.63 years). According to the international classification of CEAR (Clinical Etiological Anatomical Pathophysiological), C2 was noted in 6 (15.8%) patients, C3 — in 13(34.2%), C4 — in 7(18.4%), C5 — in 9(23.7%), C6 — in 3 (7.9%). All patients was treated with ultrasound duplex angiography (USD) using TOSHIBA Nemio XG (Japan), equipped with a convex sensor with an operating frequency of 3.5–5 MHz and a linear sensor with the range of 7.5–12 MHz. Domestically produced electric welder “Swarmed” EK-300М1 were used as a feeding device “Swarmed” EK-300М1. In manual mode, 15 sec. Parametric methods were used for statistical processing of the received results. Average arithmetic meanings (M), standard error of arithmetic mean (m) were calculated. The authenticity of the differences were evaluated using the Student`s criterion. Differences were recognized as statistically significant in p <0,05. Electric welding of the thrombosed vein segment was performed using an endovascular electric welding catheter (EC). With ultrasound examination of the thrombosed vein segment with acute ascending thrombophlebitis of the GSV the average diameter of the vein to effect by EVEW was 9.6±0.4 mm. Multiple hyperechogenicity thrombotic masses were visualized in the vein lumen. On the first day after the EVEW, the diameter of the GSV was increased by 1.3 times and averaged was 12.2±0.4 mm (p<005). The diameter of the vein increased due to the edema of the venous wall after the EVEW. After 1 month, the vein decreased in diameter significantly compared to the previous indicator and its average diameter was 7.4±0.5 mm (p<005), the vein lumen had a hypoechogenic character. After the 3 months period, compared to the 1 month after the EVEW, the vein diameter continued to decrease to 4.7±0.5 mm (p<005). After 6 months from the moment of the EVEW, the diameter of the vein was even more reduced compared to the period of 3 months — 2.4±0.4 mm (p<005). After the period of 1 year or more duplex ultrasound picture showed the completion of the process of obliteration with total vein ablation. In the observation period of 3-6 months, the occurrence of GSV repatency was observed for 3 (7,89%) patients. The ultrasound picture in the thrombosed vein after the endovascular high-frequency electric welding undergoes changes from anechogenic, hypoechogenic to echogenic character, reflects the occlusion process, obliteration and total ablation of the vein.