Abstract: The effects of grain size on the performance of BaTiO₃-based thin-layer Multilayer Ceramic Capacitors (MLCC) was investigated. A comprehensive and systematic study was conducted using Raman spectroscopy, temperature coefficient of capacitance (TCC) curves, bias characteristics, volt-ampere (I-V) characteristic curves, variable temperature impedance spectroscopy, Weibull distribution of breakdown voltage (BDV), and highly accelerated life testing (HALT) to assess the effects of grain size on the electrical properties and reliability of MLCCs. It was found that grain size plays a significant role in the bias stability and reliability of MLCCs. Fine-grained MLCCs, due to their higher grain boundary density, exhibit increased grain boundary and interface activation energy, as well as enhanced Schottky barriers. These factors contribute to a notable improvement in the breakdown strength and resistance to aging, thereby enhancing the overall reliability of the devices. The findings provide valuable theoretical insights and technical guidance for improving the performance of domestically produced MLCCs.