The microstructure evolution during sintering of the W-5 wt.%Cu nanocomposite powders was investigated for the purpose of developing a high density W-Cu alloy. The W-5 wt.%Cu nanopowder compact, fully-densified during sintering at 1623 K, revealed a homogeneous microstructure that consists of high contiguity structures of W-W grains and an interconnected Cu phase located along the edges of the W grains. The Vickers hardness of the sintered W-5 wt.%Cu specimen was $427{pm}22$ Hv much higher than that ($276{pm}19$ Hv) of the conventional heavy alloy. This result is mostly due to the higher contiguity microstructure of the W grains compared to the conventional W heavy alloy.