The effect of cobalt (0.5 wt%, 1.0 wt%, 1.5 wt% and 2.0 wt%), as an alloying element, on the microstructure and mechanical properties of W-Ni-Fe tungsten heavy alloy (WNF) prepared through spark plasma sintering (SPS) process is investigated in this work. The sintering is performed at 1400 °C with a heating rate of 100 °C/min and holding time for a period of 2 min. The properties of the cobalt added heavy alloys (WNFC) are found to be superior to that of the tungsten heavy alloy without cobalt addition. The 1.0% cobalt alloy is observed to give higher yield and tensile strengths compared to other alloys. As the mechanical properties of the alloys depend on the microstructural features, a detailed study on the influence of the microstructural parameters such as average grain size, contiguity and matrix volume fraction on the properties of the alloys is carried out. The average tungsten grain size of WNF alloy is 12.3 µm and that of WNFC alloys is from 9.5µm to 11.5 µm. The control of grain size is significantly evident in the case of spark plasma sintered alloys. The yield strength is found to be influenced by the W-grain size of the microstructure. The contiguity of the WNFC alloys is observed to decrease with increase in percentage of cobalt addition. The fractograph analysis of the tensile tested specimen helps in understanding the tensile behaviour of the alloys. The WNFC alloys show predominantly W-grain cleavage fracture compared to the WNF alloy, possibly due to the good cohesive strength of the matrix phase and W/matrix interface, because of cobalt addition and also due to the high heating rate followed in the SPS process. © 2017 Elsevier B.V.