Pure tungsten and solid solution strengthened W-Nb alloy were fabricated by selective electron beam melting (SEBM). The microstructure and crack were analyzed. Results showed that columnar crystal structure along the building direction of SEBM formed both in the pure tungsten and W-Nb alloy. The addition of Nb reduced the average size of columnar crystals from 109.78μm to 25.10μm. No significant cracks formed in the pure tungsten, but microcracks along the grain boundaries were found in W-Nb alloys. The rapid solidification, rapid cooling and holding at elevated temperature process in SEBM caused recovery and recrystallization of W and W-Nb alloys, so that the accumulated stress during the forming process was released. The cracking of W-Nb alloy is mainly formed during solidification, that is, because the liquid metal cannot feed the dendrite in a very short time during the solidification process, nanopores formed and gathered at the grain boundaries, and caused cracks along the grain boundary under little stress.