Understanding the properties of the cracks can give detailed insight about the health and status of any structure. Some cracks may not be detrimental, others may cause the structure to collapse if not inspected, recognized and repaired ahead of time. This article is about detecting the depth of a crack using laser spot thermography. A 3D finite element analysis of a laser beam as a heat source and steel specimen with cracks of various depths is performed by using COMSOL Multiphysics 5.5. Then the relationship between the crack depth and the temperature differential index is studied by using a regression analysis. Finally, the equation obtained from the regression analysis is used to predict the depth of the arbitrary cracks. The predicted depth is verified with their actual depths. The results are accurate with the error ranging from +0.3 mm to -0.2 mm.