In this paper we present the first direct observation of the microstructural evolution of metal powders during sintering using synchrotron radiation computed microtomography. Two powder materials have been investigated: loose copper powder and compacted steel powder. Neck formation and particles rearrangement have been observed in loose Cu powder. The evolution of interfaces between particles is evidenced for the steel powder as well as their partial disappearance during sintering. Moreover the anisotropy in shrinkage, induced by prior compaction, is clearly observed for the steel compact. Thanks to this information, the mechanisms responsible for this anisotropy are discussed.