Introduction: The aim of this study was to assess the efficacy and safety of endovenous thermal ablation (EVTA) in treating large diameter, >= 12 mm, incompetent great saphenous vein (GSV) in comparison to smaller ones. Methods: A retrospective comparative study was undertaken including 196 patients (205 limbs) undergoing EVTA. According to maximum GSV diameter patients were divided into two groups (group A <12 mm, group B >= 12 mm). Primary outcome was anatomic success defined as absence of reflux of GSV. Secondary outcomes were complications, postoperative pain using the 10-cm Visual Analog Scale (VAS) and improvement of Venous Clinical Severity Score (VCSS) and Chronic Venous Insufficiency Quality-of-Life Questionnaire (C Iota VIQ-20) scores assessed at 7- days, 1 month, 12 months and 24 months postoperatively. Results: 118 patients with GSV diameter <12 mm (group A) and 87 with GSV diameter >= 12 mm (group B) were included. Patients' demographics, CEAP classification and length of ablated vein did not differ between the groups. Preoperative VCSS and VAS pain score were significant greater in group B (6.03 vs 6.94, p = .04 and 5.21 vs 5.77, p = .032, respectively). No differences in adverse events were observed post-operatively among groups. GSV occlusion rate at 1 month was 98.3% (SE 1.3%) in group A and 96.5 % (2.2%) in group B (p = .3), at 12 months 95.7% (SE 2%) and 94.2% (SE 2.8%) (p = .5), and at 24 months 94% (SE 2.4%) and 93.1% (SE 3%) (p = .4) respectively. Both groups experienced significant and similar improvement in their VCSSs and CIVIQ scores postoperatively. In a subgroup analysis among different EVTA and GSV >12 mm, 1470 nm endovenous laser ablation (EVLA) and radiofrequency ablation (RFA) showed comparable results in terms of occlusion rates, complications, VCSS and CIVIQ scores. Conclusions: Endovenous thermal ablation techniques are efective and safe in the treatment of GSV incompetence regardless the diameter of the GSV. Both 1470 nm EVLA and RFA techniques performed similar outcomes.