The advancement of thermoelectric materials is vital for efficient energy conversion. However, reducing thermal conductivity and maintaining the electrical properties is still challenging and limits the overall thermoelectric material’s efficiency. This study investigates the impact of the sintering effect on the grain size of Al-doped ZnO/Graphene nanoparticles via the microwave-assisted sol–gel method. The composition of the materials was confirmed using X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDX), including the successful integration of Graphene. Field emission scanning electron microscopy (FESEM) revealed the critical role of sintering at 1100 °C, which reduced the particle sizes of Zinc Oxide to 392.987 nm and Aluminum to 48.57 nm. This reduction in particle size is essential for decreasing thermal conductivity and optimizing the figure of merit (ZT) values. Furthermore, this approach not only enhances thermoelectric properties but also preserves the stable morphology of Graphene, ensuring consistent electrical conductivity. These findings suggest that this method promises improved performance and higher ZT values, marking a significant step forward in thermoelectric material development. Future applications may benefit from these enhanced properties, leading to more efficient energy conversion systems.