Abstract: In order to ensure the coolant system pipes in the nuclear reactor run well in in-service condition, detecting fatigue crack height of base metal in inner and outer walls effectively and accurately was necessary. This paper mainly introduced the detection principle, test methods, test block designing and UT transducer parameter selection of fatigue crack height of base metal. By a great deal of testing and field inspection, it proved that through fabricating corresponding reference block and performance verification test block according to the pipes sizes, and using 5 MHz 45° shear wave angle probe, the crack height which was larger than 2 mm could be measured by using ultrasonic tip diffraction method.