Thin film coatings, and materials is greatly influenced by imperfections such as surface and interface roughness and surface or subsurface damage (SSD). The deposit of thin films requires well polished substrate. In most cases, the grinding process is the first step of the brittle material machining, where SSD is produced. Therefore, the generated SSD depth in this step is very important and influences the time of succeeding operations and therefore the cost of optical components and their qualities. Moreover, during lapping and polishing, SSD must be removed in order to improve the quality of the thin film that will be deposited on the substrate. This study inspects and measures the SSD of brittle materials developed in lapping process through a bonded interface sectioning technique. A series of experiments is conducted to reveal the influence of lapping parameters on SSD depth and surface roughness. Results indicate that SSD depth and surface roughness are mostly sensitive to abrasive grain size. The ratio of SSD depth to surface roughness (peak to valley) is confirmed to be (2.92 0.75), which can predict the SSD depth of brittle materials lapped by different abrasive grain size with a rapid roughness measurement.