In this work, we report a facile strategy to prepare polyamide 6/exfoliated graphite nanoplate (PA6/xGNP) nanocomposites by in situ exfoliation of expanded graphite (EG) via ultrasound‐assisted extrusion. The EG was preliminarily exfoliated by ultrasonication, and then blended with PA6 and further exfoliated into graphite nanoplates during the ultrasound‐assisted extrusion process. Compared with chemical exfoliation of EG, ultrasonic exfoliation minimized defects in the carbon layers. Meanwhile, the in situ exfoliation of EG via ultrasound‐assisted extrusion prevented the reagglomeration of xGNPs in the melt blending process. The well‐dispersed xGNP structure significantly improved the mechanical performance of PA6 composites; with an xGNP loading of only 0.5 wt%, the tensile and flexural strengths of the PA6 nanocomposite increased from 65.4 and 67.8 MPa to 90.5 and 91.2 MPa, respectively. The heat deflection temperature also greatly improved from 76.5°C to 115.4°C by adding 1 wt% xGNPs. These results demonstrate a promising approach for large‐scale fabrication of high‐performance polymer/xGNP nanocomposites. POLYM. ENG. SCI., 58:1739–1745, 2018. © 2017 Society of Plastics Engineers