Abstract: A method for defect characterization and evaluation using mono-pulse ultrasonic (MU) reflection behavior in pultrusion composites beam (PCB) was proposed. Different pultrusion composite specimens were designed and prepared. A MU testing system was constructed. The acoustic relationships among time-domain features, image features of MU signal and internal defect behavior of PCBs were investigated. A MU method for characterization and evaluation of internal defects of PCBs was established. The experimental results showed that MU reflections in PCBs had very good time-domain discernable signal relation and image mapping relationship with defects therein. The internal defects characterization, detection, and qualitatively and quantitatively evaluation of PCBs can be realized effectively using MU reflections from PCBs. The surface detection dead-zone reached a single prepreg ply thickness (about 0.125 mm). It provided a very effective ultrasonic characterization and evaluation method for PCBs. Good practical application had been obtained.