PC-MUSIC (phase-coherent multiple signal classification) was introduced to study the defect quantification based on the super-resolved ultrasonic image. The ultrasonic array data can be collected via full matrix capture process, and pre-processed in time domain to extract the scattered signals related with defect. The scattered signals were post-processed by PC-MUSIC to obtain the ultrasonic image with super resolution. The ultrasonic image was analyzed to extract the lateral cross section, the -6 dB main lobe width of which was defined as the assessed length of defect. The experimental system had been built, and a block of Al with a 10mm-long slot which had been machined was chosen as the tested object. It is shown that the PC-MUSIC can accurately assess the length of defect with an error less than 10% when the proper dimension of signal subspace.