Conventional B-mode imaging systems that are clinically used for measurement of arterial diameter are expensive, non-portable and non-scalable, demand operator expertise and are not field amenable. To address this, we have previously developed a portable image-free ultrasound technology, ARTSENS®, that uses A-mode scans for easy and fully automated assessment of arterial wall dynamics. Earlier algorithms of ARTSENS® to evaluate arterial diameter were based on fit-models of the lumen well which does not directly represent the distance between the intima-lumen boundaries of the arterial walls. In this work, we have proposed a method that uses the unwrapped analytic phase of the A-mode echoes to directly identify the locations of the intima-lumen boundaries of the near and far wall for diameter estimation. The performance of the method was evaluated using systematic simulation and in-vitro phantom experiments. For the in-vitro experiments, a B-mode ultrasound system was used as the reference. Simulation experiments have demonstrated that the proposed method yields accurate measurements even at an SNR of 3 dB for the A-mode scans. Further, the phantom experiments have demonstrated the measurement repeatability, where the coefficient of variability for the beat-to-beat diameter measurements was less than 0.8%. The diameter measurements performed by the proposed method, when compared to the corresponding B-mode measurements, exhibited a strong correlation (r2 = 0.93, p < 0.05), yielding an insignificant (p = 0.35) bias. The RMSE and maximum percentage error were 0.01 mm and 1.88% respectively, validating the measurement accuracy on the phantom. © 2019 IEEE.