Abstract: This paper proposes an effective method of using ultrasonic phased array to measure bottom cracks from single-sided-unilateral. The heights of artificial cracks in different thickness blocks are measured by the Absolute Arrival Time Technique (AATT) with ultrasonic phased array in testing simulation and experiment. The height of natural crack in a weld is also measured by AATT. A comparative analysis of the experimental results and the conventional TOFD measurement results is conducted, and the analysis results show that the height of bottom crack can be measured quantitatively by AATT from one side and this measurement technique can achieve the same accuracy as TOFD measurement.