Densification, microstructures and dielectric properties of CaO–MgO–SiO glass-ceramics were investigated with ZrO nucleating agent using two-stage heat treatment in the low temperature co-fired ceramic (LTCC) method. The amorphous cullet (abbreviated as CMSZ glass) was synthesized by mixing CaO, MgO, and SiO powders with ZrO nucleating agent, which were melted at 1500 °C before the quenching process. The results show that CMSZ glasses annealed below the critical nucleation temperature (T) and sintered at 950 °C exhibit obvious bloating phenomena. On the other hand, CMSZ glasses annealed above T demonstrate that zirconia nucleation occurs at the first-stage heat treatment (annealing). The second-stage heat treatment (sintering) is characterized by the rapid crystal growth of CaMgSiO phase around the ZrO crystallites, indicating growth of nuclei over the critical size above T. Moreover, specimens fabricated using two-stage heat treatment and annealed above T exhibit appropriate microwave dielectric properties and high mechanical strength.