STUDY OBJECTIVE: To determine whether the long-axis in-plane (LAX-IP) combined with short-axis out-of-plane (SAX-OOP) technique is more suitable than modified dynamic needle tip positioning (MDNTP) technique for ultrasound-guided radial artery catheterization in infants. DESIGN: A randomized controlled trial. SETTING: Department of Anesthesiology, Children's Hospital of Chongqing Medical University. PATIENTS: Overall, 72 patients, aged 1-12 months old, who were primarily undergoing thoracic or cardiac surgery in the Children's Hospital of Chongqing Medical University between July 1, 2021, and March 31, 2022, were selected. These patients were randomly divided into two groups: i) the MDNTP group and ii) the LAX-IP combined with SAX-OOP group. INTERVENTIONS: Radial artery cannulation in the two groups was performed using ultrasound-guided MDNTP or LAX-IP combined with SAX-OOP technique. MEASUREMENTS: The primary outcome was first-time success rate, and the secondary outcomes included total success rate, cannulation time, and incidence of complications. MAIN RESULTS: In the LAX-IP combined with SAX-OOP group, the first-time success rate was 75.0% (n = 27), total success rate was 97.2% (n = 35), cannulation time was 91.39 ± 102.60 s, puncture attempts was 1.5 ± 1.3 times, and local hematoma was formed on the first day in one (2.8%) infant. In the MDNTP group, the first-time success rate was 36.1% (n = 13) (P = 0.001; RR, 2.08; 95% confidence interval, 1.29-3.34), total success rate was 91.7% (n = 33) (P = 0.303; RR, 1.06; 95% confidence interval, 0.95-1.19), cannulation time was 181.00 ± 146.72 s(P = 0.047; Median difference,-89.61; 95% confidence interval, -149.12 to -30.10), puncture attempts was 2.3 ± 1.6 times (P = 0.133; Median difference,-0.81), and local hematoma was formed on the first day in nine (25%) infants (P = 0.006; RR, 0.11; 95% confidence interval, 0.01-0.83). No thrombosis occurred in any group. CONCLUSIONS: The ultrasound-guided LAX-IP combined with SAX-OOP technique for radial arterial catheterization in infants, which was performed by anesthesia residents, exhibited an increased first-time success rate, reduced cannulation time, and lower incidence of complications than the MDNTP technique.