In order to improve the inspection efficiency of Ultrasound Lock-in Thermography (ULT) and to obtain optimal results, the ultrasound modulation, thermal sequence image processing and the optimal parameter selection were investigated.The principle of ULT was analyzed theoretically,then an inspection system was developed,and several experiments were carried out on the steel plane with surface (subsurface) microcracks and Al alloy with defects at different depths.In inspection, the modulated ultrasonic square signals were used to excite the samples and an infrared camera to collect the transient sequences,then the lock-in processing was taken to obtain phase images and amplitude images.Experimental results indicate that, for given A3 steel samples, the modulation frequency of 0.5 Hz and the pressure between sample and ultrasonic transducer about 0.25 kN is a good choice.Furthermore, the distance between the injection position and the crack has a few effects on the inspection and appropriate injection position should be depended on the specific structure of samples.As deeper defect can be detected with decreasing modulation frequency, the defect at 4 mm depth on the Al alloy sample is detected at modulation frequency of 0.3.Moreover, the method can also be used for detecting the skin unbond on a honeycomb sandwich structure.Using the inspection system, reliable detection of contacting interface-type defects can be done for a few seconds.