Porous magnesium/carbon nanotube (Mg/CNT) composites with various CNT concentrations (0.05 and 1 wt%) and overall porosities (20, 30, and 40 %) were synthesized by a powder metallurgical method and characterized to study their microstructure and yield strengths. The introduction of pores leads to the reduction of material strength. Thus, it is attractive to add reinforcement phase to produce porous composites and improve yield strength. The porous Mg/CNT composites with low overall porosity have smaller average pore size than the porous Mg/CNT composites with high overall porosity. The compression testing results showed that the average yield strength is about one to four times of the yield strength of sand-cast Mg. Yield strength of porous Mg-0.05 wt%CNT composites is not significantly different from that of porous Mg-1 wt%CNT composites, when both composites have the same overall porosity. The average yield strength along the in-plane direction is larger than that along the normal direction for each type of porous Mg/CNT composite. The Gibson and Ashby model was used to estimate yield strength and the results show that the theoretical estimations of yield strength agree well with the experimental yield strength values for the porous composites with high overall porosity (i.e., 30 or 40 %), while the estimations are lower than the experimental data for the porous composites with low overall porosity (i.e., 20 %).