  The nature of the internal thermal X-ray emission seen in ``thermal
composite" supernova remnants is still uncertain. Chandra observation of the
3C391 shows a southeast-northwest elongated morphology and unveils a highly
clumpy structure of the remnant. Detailed spatially resolved spectral analysis
for the small-scale features reveals normal metal abundance and uniform
temperature for the interior gas. The properties of the hot gas comparatively
favor the cloudlet evaporation model as a main mechanism for the ``thermal
composite" X-ray appearance, though radiative rim and thermal conduction may
also be effective. A faint protrusion is found in Si and S lines out of the
southwest radio border.
