Native arteriovenous fistulas (AVFs) remain the preferred vascular access for hemodialysis, yet early non-maturation continues to limit their clinical utility. Preoperative vessel diameterparticularly cephalic vein and feeding artery calibrehas emerged as a potential determinant of functional maturation, though its precise impact across different AVF configurations remains incompletely defined. This study evaluates the influence of preoperative venous and arterial diameters on maturation and early patency of radiocephalic and brachiocephalic AVFs.  Methods: A prospective cohort of 120 patients undergoing native AVF creation was assessed using standardized preoperative Doppler ultrasound to measure cephalic vein and inflow artery diameters. Patients were followed for a minimum of three months. Functional maturation was defined as successful use of the AVF for six consecutive hemodialysis sessions. Demographic variables, comorbidities, fistula type, anastomotic technique, and preoperative vessel metrics were analyzed using chi-square and independent t-tests, with statistical significance set at p < 0.05. Results: Overall maturation occurred in 70.8% of AVFs. Brachiocephalic fistulas demonstrated significantly higher maturation rates than radiocephalic fistulas. The strongest predictor of successful maturation was cephalic vein diameter, with mature AVFs showing a mean diameter of 2.80 ± 0.32 mm compared with 2.31 ± 0.35 mm in non-matured AVFs. Arterial diameter did not significantly differ between groups. Age, sex, diabetes, hypertension, ischemic heart disease, peripheral vascular disease, and previous dialysis access showed no significant relationship with maturation. Anastomotic technique also had no measurable impact. Conclusion: Preoperative cephalic vein diameter is the key determinant of AVF maturation, while arterial diameter and patient comorbidities exert minimal influence when suitable veins are present. Routine ultrasound vessel mapping and diameter-based access planning may enhance early AVF success and reduce catheter dependence, supporting broader adoption of anatomy-driven access strategies.