The on line inspection of small diameter tubes is studied by using torsional guided wave T(0,1). Considering the service environment of tubes and their structural characteristics, a novel transducer array applicable to on line inspection of tubes is developed. The transducer employs shear piezoelectric ceramic as the sensitive element, and has the characteristics of flexible and lightweight. Experimental results show that a ring of transducers can efficiently excite the T(0,1) mode guided wave in tubes. By extracting each of guided wave modes information in echo signals, the experimental system can effectively identify the state of the defects in tubes. For the tubes that contain support structure, comparing and analyzing the changes of scattered signal and echo signal from characteristic structure in tubes is contributed to identify the generation and extension of defects near the support region.