Typical sintering experiments were conducted to understand the spark plasma sintering (SPS) mechanisms. Based on the results of the direct visual observations and characteristic microstructure analysis, we believe that spark discharge does indeed occur during the SPS process. The high-temperature spark plasma could be generated in the microgaps due to the discharge effect. Fast and efficient sintering can be achieved under the combined action of spark discharge, Joule heating, electrical diffusion and plastic deformation effect in the SPS process.