In this study, the simultaneous effects of modification Mg₂Si reinforcement particles in Al-Mg₂Si in-situ composite by erbium addition, along with hot compression testing at temperatures of 300, 350, 400, and 450 °C and strain rates of 0.001, 0.01, 0.1, and 1 s⁻¹, were investigated. Microstructural evolutions were examined using optical and scanning electron microscopy, while thermomechanical characteristics were analyzed based on Arrhenius constitutive equations. The results revealed that the addition of 0.5 wt.% erbium transformed the morphology of primary Mg₂Si particles from dendritic to polygonal, reducing their average size from 56 μm to 15 μm. The lamellar morphology of eutectic Mg₂Si particles also changed to short fibrous and particulate forms due to the modification of interfacial energy between the α-Al and Mg₂Si phases. Furthermore, thermomechanical processing accelerated microstructural refinement through mechanisms such as mechanical fragmentation, particle spheroidization, and dynamic recrystallization. By analyzing the flow curves and performing linear fitting of strain rate, temperature, and peak flow stress values, the constitutive constants were determined, and their variations with processing parameters were evaluated. The activation energy for hot deformation was calculated to be 259.14 kJ.mol-1. Finally, the accuracy of constitutive equations was validated by comparing the experimental (measured) flow stress values with those predicted by the model.