Infrared Thermography is a Nondestructive Testing and Evaluating (NDT&E) technique that allows the non-contact inspection of systems and materials through a mapping of thermal patterns on the surface of the objects of interest. Defect detection principle in active thermography is based on the fact that a difference of thermal properties exists between the sound area and a defective region, which can be used for defect detection and quantification purposes. In this paper, experimental studies were made on the applicability to the detection of artificial defects in a steel flat plate with 12 flat-bottomed holes with three different sizes located at three different depths. The object was periodically heated by quartz lamps combined with light controller. The controller was operated by the same reference signal for the lock-in thermography. It was found that the defects were detected by localized contrast change in the phase delay images. It was also found that the location and size of defects can be estimated by the area of contrast change in the phase delay images which was clearly observed compared with conventional thermography techniques. The relationship between the values of phase delay and heating periods or modulation frequency was examined for several defect depths.