Infrared lock-in thermography is a novel non-destruction testtechnique (NDT). It allows for defect detection in subsurface of structures. This technique depends on transmission and reflection of periodic heat flux which is launched into the inspection structure. Defects can be indicated by comparing phases of surface temperature. Heat conduction model is utilized to describe the transmitting process of sinusoidal (cosinusoidal) modulated heat flux in metal plates. Effect of modulation frequency on detection depth and detectivity is investigated. Experiments are carried out to inspect metal plates with artificial defects in room-temperature. A sinusoidal (cosinusoidal) modulated lamp is used as active heat source. Image sequences are collected by infrared camera and processed to obtain lock-in phase image by Cedip Infrared System. In frequency domain, phase images can be used to analyze the effects of modulation frequency on detecting defects in different size and depth. Experiments indicate that the detectivity will be reduced if the modulation frequency is lowered in order to detect deeper defects. A method is proposed to achieve deeper detection depth and higher detectivity, in order to obtain accurate results.