To compare the ultrasound characteristics between functional, mature arteriovenous fistulas and functional, non-mature arteriovenous fistulas and to identify the predictors of arteriovenous fistula maturation in the forearm. Patients with newly set-up functional arteriovenous fistulas were enrolled in this prospective cohort study. Ultrasound examinations were conducted pre-operatively and post-operatively. The inner vessel diameter, blood flow volume, and resistance index were measured and compared between the maturation group (Group M) and non-maturation group (Group N). Baseline parameters were calculated to determine the predictors of non-maturation of arteriovenous fistulas. All 52 patients with functional arteriovenous fistulas, who were categorized into Group M (25 patients, 48.08%) and Group N (27 patients, 51.92%), finished 24 weeks of follow-up after arteriovenous fistula surgery. The arteriovenous fistulas displayed a significant and rapid increase in the vessel diameter (mean increase of 1.34 times in the arteries and 1.92 times in the veins) and blood flow volume (mean increase of 9.29 times of the arteries and 43.66 times of the veins) and a decrease in the resistance index (mean decrease in 48.00% in the arteries) 8 weeks after surgery. Group N had a lesser increase in the vessel diameters (1.78 times vs 2.06 times, t = -3.136, p = 0.003) and blood flow volume (33.98 times vs 54.11 times, t = -2.383, p = 0.021) of the cephalic vein draining segments (a6) than Group M. The baseline diameter of a6 was the only independent predictor (regression coefficient = 26.229, p = 0.008) of maturation of the functional arteriovenous fistulas after correcting for sex, age, diabetes kidney disease, weight, and height. The baseline diameter of the cephalic vein was the only predictor of arteriovenous fistula maturation based on the pre-operative ultrasound measurements in Chinese hemodialysis patients.