Abstract: The failure analysis of thin-walled stainless steel small diameter tube was carried out on the sample tube cut from the cracked reheater tube of a power plant. Material analysis showed that the material mark of reheater tube is 10Cr18Ni9NbCu3BN and weld material is YT-304H. Penetration testing found that cracks were produced and expanded on heat affected zone close to one side of the parent metal. Microstructure analysis found that there existed intergranular corrosion in the weld fusion zone. To prevent tube-burst accident happening again in the short term,the test scheme by phased array ultrasonic testing and the reference blocks for the test of reheater tube were performed. Six defective tubes were found within the scope of 56 samples cut from the power plant. All cracks of the defective pipes were verified by penetration testing,and there was no trace of the crack being found among the rest of the tubes by penetration testing method. The results tested by phased array ultrasonic testing and penetration testing were validated with each other.