The sintering behavior of the powder injection molded Fe micro-nanopowder fabricated under low temperature and low pressure conditions of 70 degrees C and 4 MPa was investigated in terms of the microstructural development. It was found that the sintered density of the micro-nanopowder was remarkably increased in the temperature range of 800 to 900 degrees C by the nanopowder effect due to the enhancement of the material transport by grain boundary diffusion. The micrograins in the micro-nanopowder almost maintained their initial grain size until the sintered density reached 90% of the theoretical density (TD), even showing no drastic grain growth under the condition of full density of 97% TD after sintering at 1250 degrees C for 3 h. This means that the presence of the nanopowders not only increases material transport paths for rapid densification but also suppresses the grain growth. The experimental findings in this study provide the potential application of micro-nanopowders for processing the PIM products with full density and fine microstructure. (C) 2013 Elsevier B.V. All rights reserved.