A qualitative assessment of the capabilities of viscoplastic model capturing the complex thermomechanical behavior exhibited by ceramic materials at elevated service temperatures is presented. Constitutive equations for the flow law and evolutionary law have been formulated based on a threshold function which exhibits a sensitivity to hydrostatic stress and allows different behavior in tension and compression. Inelastic deformation is treated as inherently time-dependent. A rate of inelastic strain is associated with every state of stress. Creep, stress relaxation, and rate sensitivity are phenomena resulting from applied boundary conditions and are not treated separately in an ad hoc fashion.