The effect of crystallization on the stress required for complete constrained sintering of a low-temperature cofirable CaO-B2O3-SiO2 glass-ceramics has been investigated under uniaxial constant and cyclic loading. As the formation of crystalline phase of wollastonite (CaSiO3) increases, the uniaxial viscosity of the porous glass-ceramics during firing increases. Moreover, the required uniaxial stress to have complete constrained sintering, i.e., zero strain or strain rate at the perpendicular directions, is in the range of 80-180 kPa, which shows no significant dependence on heating rate and firing temperature. The measured data of required stress are close to those calculated using the viscous analogy for the constitutive relationship of a porous sintering compact.