Lightweight (Ca1-x, Mgx)Zr4(PO4)6 (CMZP) ceramics (X = 0, 0.1, 0.2 and 0.3) have been fabricated with relative densities of less than 0.35 by the polymer foam technique, and higher than 0.35 by the polymer powder technique. The polymer powder method forms an inhomogeneous pore structure having an average pore size of 30-80 microns compared with the polymer foam method, which yields large and uniform pore structures with pores 250-300 microns in diameter. Gas permeability of these two ceramics was examined and distinct behaviors were exhibited. The lightweight CMZP ceramics showed flexural strength comparable to that of lightweight ZrO2, but lower in compressive strength. The excellent resistance to thermal shock makes the lightweight CMZP ceramics a strong candidate for applications in severe environments.