We present ROSAT PSPC and ASCA observations of the supernova remnant (SNR) W28. The overall shape of X-ray emission in W28 is elliptical, dominated by a centrally-concentrated interior emission, sharply peaked at the center. The ASCA spectra reveal emission lines of Ne, Mg, Si, and Fe K
continuum extending at least up to 7 keV, showing thermal origin with a hot
thermal component. We found that spectral variations are present in W28. The
southwestern shell can be fit well by a plane-shock model with a temperature of
1.5 keV, and the northeastern shell, with a lower temperature of 0.56 keV.
Unlike for the southwestern and northeastern shells, the central emission
requires a two-temperature components with 0.6 keV and 1.8 keV. The low
temperature component is similar to those seen in other Mixed-morphology SNRs.
The X-ray luminosity of W28 is 6x 10^34 ergs/s, and the estimated X-ray mass is
only ~20 - 25 solar mass. A comparison of W28 with other typical
Mixed-morphology SNRs reveals significant differences in its X-ray properties;
W28 has a significantly higher temperature and noticeable spectral variations.
W28 belongs to a class of SNRs considered by Chevalier (1999), with a radiative
shell interacting with clumpy molecular clouds. X-ray emission at its center is
a ``fossil'' radiation from gas which was shocked early in the evolution of the
remnant, and its centrally-peaked morphology could have been caused by
processes such as evaporation, electron thermal conduction, and mixing induced
by various hydrodynamical instabilities. But W28 poses a challenge for existing
models of X-ray emission, because the evaporation model of White & Long (1991)
is in conflict with observations, while the presence of temperature variations
seems inconsistent with SNR models with efficient thermal conduction.