<p>This study investigates the optimization of fracture toughness properties in 3D-printed polylactic acid (PLA) reinforced with milled carbon fiber composites using the material extrusion (MEX) based 3D printing technique. A Taguchi L₁₆ orthogonal array design was applied to systematically examine the influence of key process parameters, including printing temperature, layer thickness, infill pattern, and print speed, on mechanical properties such as critical load, critical stress, fracture toughness, and strain energy release rate. Grey relational analysis (GRA) was employed to determine the optimal combination of MEX parameters, enhancing multi-objective optimization. The novelty of this work lies in the combined use of GRA and Taguchi methods for optimizing multiple fracture-related properties in PLA/carbon fiber composites produced via MEX. This integrated approach offers a practical framework for improving the mechanical integrity of composites and supports the advancement of high-performance, additively manufactured components for structural and engineering applications. The results revealed that a printing temperature of 240°C, a layer height of 0.4 mm, a grid infill pattern, and a print speed of 30 mm/s yielded the highest Grey relational grade (GRG), indicating superior fracture toughness performance. The findings provide a systematic approach for improving the mechanical properties of PLA/carbon fiber composites and contribute to advancing material extrusion-based additive manufacturing for high-performance engineering applications.</p>