Abstract: Based on infrared lock-in thermography technology, the nondestructive testing of composite grid stiffened structures with typical defects was carried out.The result was also analyzed using infrared lock-in thermography system developed by Cedip in France.The effects of loading frequency and output voltage offset to the testing result were discussed.The result shows that there are more information of phase figures than that of amplitude figures. Different frequencies will lead to different results and the loading frequency is the key to the success.Increasing the output voltage offset will make a useful contribution for better testing.This method is feasible to inspect the unknown defect of composite materials.