The pore structure of iron reduced from wustite by hydrogen gas is characterised by measurement of pore volume and critical pore radius using mercury penetration porosimeter. By utilizing partial reduction and one hour annealing, morphology of the reduced iron is thoroughly investigated by both optical and scanning electron microscopy. Summerized results are as follows: 1) The morphological changes in reduced iron layer are closely related to the length of sintering time. 2) Swelling of reduced iron is basically due to the crystal growth of γ-iron reduced from wustite. 3) In the temperature range of 700-900℃, the critical pore radius is described by the relationship. Critical pore radius＝A·exp(-9300/RT). 4) The growth rates of the critical pore radius vary with crystal structure. In the α-iron range, the rate of growth is extremely slow compared to that occurring in the γ-iron range.