Ultra-low thermal expansion ceramics based upon CaZr 4 (PO 4 ) 6 -Li 2 O system were synthesized through solid state sintering. Lithium oxide was introduced to the CaZr 4 (PO 4 ) 6 lattice to promote liquid phase sintering to form CaZr 4 (PO 4 ) 6 with strongly negative thermal expansion. A small addition of Li 2 O has improved the densification rate, but also caused an excessive grain growth, which resulted in extensive microcracking. The microcracking lowered the bulk thermal expansion to -6x10 - 6 o C - 1 . Excessive amount of Li 2 O content also led to formation of the secondary phase, Li 2 Zr(PO 4 ) 2 , and also caused the swelling of the CaZr 4 (PO 4 ) 6 phase to lower the sintered density. An addition of over 5% of lithium oxide, the glass phase surrounding the CaZr 4 (PO 4 ) 6 grain, together with the secondary phase increased the thermal expansion. At 2.5% addition, there was an indication that the viscous glassy phase has partially relieved the stresses associated with the thermal expansion anisotropy during cooling from the processing temperature. It was demonstrated that the microstructure of the CaZr 4 (PO 4 ) 6 could be altered by adding varying amount of lithium oxide to tailor the thermophysical properties of the material.