Fabrication of pure magnesium (Mg) disk was performed by powder metallurgy with the compaction method of spark plasma sintering (SPS). The effect of mechanical milling time on the microstructure, density, and porosity of the disk specimen was investigated. At an identical temperature, the 4 and 5 h milled specimens exhibited a nearly overlapped displacement curves during SPS, and a higher value indicating a higher densification degree than that of the 3 h milling powder. In agreement, the specimen density increased consecutively from 1.76 to 1.77 and 1.80 g/cm3 for the milling time of 3, 4, and 5 h. However, the porosity increased from 1.21% to 1.49% when the milling time increased from 3 to 4 h and further to 3.44% for 5 h milled specimens. The microstructure observation revealed that the average grain size decreased, and the pores became smaller and elongated with increasing milling time. The number of pores was higher with the gain fraction of grain boundaries. The 3 h milled specimen contained the highest atomic fraction of oxygen (21.9 at%) than that of the 4 and 5 h milled specimens (5.6 at% and 7.9 at%). The optimum milling time for obtaining high density and low porosity of pure Mg disk was 4 h.