Ultrasonic Test(UT) technology for dielectric defects of Multilayer Ceramic Structure(MCS) is studied. Firstly, UT is proved to be applicable and effective to voids and delamination through thin layer reflections theoretical calculating on Multi-Layer Ceramic Capacitors(MLCC) structure, as low UT frequency can get very strong reflection signal at extremely thin aperture. Simultaneously, suggestion of C-Scan testing parameters including equivalent focal length, surface wave time, data gate and gain is given. Then, the practical UT test of MCS indicates that 50 MHz is the optimum screening frequency. Finally, defect determining and locating is accomplished via A-Scan and transit time respectively, the conclusion is also verified by sectioned UT sample preparation and detection.