Highly densified MgB2 superconductors were successfully fabricated using a spark plasma sintering (SPS) technique, and their superconductivity with respect to microstructural evolution was evaluated. Full densification with final density close to the theoretical density was achieved at a temperature of 1000 degrees C within a total SPS processing time of 40 min. Both an MgB2 specimen sintered at 1000 degrees C for 30 min and one sintered at 1050 degrees C for 10 min exhibited a high critical transition temperature (T-c) similar to that of an MgB2 single crystal (39 K), and a very sharp superconducting transition width (Delta T) less than 0.5 K. In addition, high critical current densities (J(c)) of 7.7 x 10(5) A/cm(2) in a field of 0.6 T at 5 K and of 8.3 x 10(4) A/cm(2) in a field of 0.09 T at 35 K were obtained. These excellent superconducting characteristics of the SPS-processed MgB2 are attributed to uniformly distributed secondary MgO phase nanoparticles and well-developed dislocations in the microstructure that may act effectively as extrinsic flux pinning sites, resulting in the strong pinning force showing a high J(c) of 8.7 x 10(4) A/cm(2) even in the condition of a field of 4 T at 5 K.