Two compaction equations (isothermal equations of state) for powders were developed, and experimemts on the compaction of various ceramic powders were designed to examine these equations. A nonlinear least-squares technique was utilized to evaluate these and other published compaction equations for press compaction of submicrometer and nanosize powders. A modified linear least-squares method was developed for fitting experimental data to one of the present equations. The root-mean-square deviation per degree of freedom (rms) for fitting compaction data of powders using the present equations was reduced by almost one order of magnitude by comparison with fitting to that of Cooper and Eaton’s equation. Data analysis indicates that our equations yield better fitting of data than the previous equations, and these equations can be applied to chemically different powders, powders with different size distribution of particles.