Various damage index (DI) algorithms of detecting changes such as a loosen bolt and a delamination development in a composite structure were examined using ultrasonic Lamb waves generated by embedded piezoelectric active sensors. The DI is a single value that is a function of response signal’s attenuation due to any damage or changes in a structure. Various DI algorithms such as active damage interrogation (ADI), time domain root men square (RMS), short time Fourier Transform (STFT) and time reversal (TR) were discussed. For experimental validation, a composite stiffened panel was used, and loosen bolt damage and low-velocity- impact damage were tested. In order to pitch and catch Lamb waves, surface mounted PZTs (lead zirconate titanate) were used. According to the DI algorithms, appropriate ultrasonic guided Lamb waves were selected for actuators. Each set of DI algorithm and drive signal showed different characteristics to detect the damage.