Complex characteristics of the pores and properties of porous reaction-bonded Si3N4 have been investigated and correlated with the microstructure of Si3N4 grains. Porous ceramics with porosities of <= 75 vol% and alpha-Si3N4 matte grains (alpha/beta phase ratio of 1.5) or alpha-Si3N4 whiskers (alpha/beta phase ratio of 0.36) were prepared by in situ nitridation of silicon powder. To obtain various microstructures by alpha -> beta-phase transformation and grain morphology modification, samples were heat-treated at 1700 degrees C while embedded in a Si3N4 powder bed containing MgO. By the growth of alpha-matte or beta-Si3N4 grains on the pore walls, highly interconnected structures with spherical cavities and unimodal pore size distributions resulted with d(50) approximate to 8.8 mu m and approximate to 6.5 mu m, respectively. In contrast, alpha-whiskers grew inside the pore cavities; thus, complex and irregular inter-particle pores appeared which generated an extra peak near d(50) approximate to 1 mu m forming a bimodal pore size distribution. Compared to the alpha-matte grains, alpha-whiskers densified upon heat treatment and produced a large drop in porosity, which resulted in a structure with less interconnectivity. As a consequence of growth of fine beta-rods, pore walls became relatively smooth and whisker free; thus, inter-cluster channels were modified to spherical cavities with d(50) approximate to 3.7 mu m. Samples exhibiting networked whiskers and fine pores or low porosity demonstrated higher compressive strength than the interconnected structures with spherical cavities.