The Einstein model to consider thermal effect in universal equations of state is modified, and an approach to solve parameters at absolute zero temperature from experimental data at a reference temperature is proposed and applied to the Baonza equation. The thermal Baonza equation is applied to study thermo-physical properties of α, β and γ-Si3N4. Through analyzing experimental data in literature, the input data-set for the three phases are determined. The results calculated from the thermal Baonza equation is in agreement with available experimental data.