A modified Campbell model has been provided considering the internal stress and the "core-shell" microstructure that occurs in X7R-type multilayer ceramic capacitors (MLCCs). The capacitance stability coefficient alpha was defined to denote the effect of a direct current (DC) bias field on the capacitance of MLCCs. According to this model, the dielectric constant will decrease with DC bias field while the alpha coefficient will increase. The dielectric properties of MLCCs with different active layers, sintering temperatures, grain size, and strontium doping concentration were measured in order to verify the validity of the model. The experimental results exhibited good agreement with the model, which provided qualitative explanations.