The detection and characterisation of subsurface flaws in nonmetallic materials are very important for people’s health, lives, and environment. Possible damage must be detected early and reliably. A capacitive approach for detecting the subsurface cracks is discussed. A uniplanar capacitive sensor with multi-electrodes for obtaining the corresponding electrical capacitance information of the measured slab is presented. An experimental rig, which is composed of a uniplanar capacitive sensor of 8-electrodes and two engineering plastic samples, has been built for damage detection of nonmetallic material. Principal component analysis is used to extract relevant features from capacitance values for damage detection and identification. The simulated, as well as the preliminary experimental results show that the current approach is capable of detecting subsurface damages of nonmetallic materials and discriminating the flaws. The proposed approach is feasible and effective for damage detection and health monitoring.