The coefficients of thermal expansion (CTE) of low-viscosity phosphate sealing glasses are too high to match low-CTE materials such as silicon. In this study, a low-viscosity SnO–MgO–P₂O₅ (SMP) glass powder was mixed with 15 vol % of a negative thermal expansion cobalt-magnesium pyrophosphate (Co_0.5Mg_0.5)₂P₂O₇ (CMP) ceramic filler to reduce the thermal expansion of the sealing material. The influences of the addition of CMP filler and the CMP particle size on the sintering, crystallization, and the resulting properties of the CMP-filled SMP composites were investigated. It was discovered that the decrease in CMP particle size decreases the effectiveness of CTE reduction of the sintered composites, the crystallization tendency of SMP glass, the softening temperature of the sintered composites, and the resistance to flow above the softening temperature.