Transmission electron microscopy has been applied to the study of interfacial reactions of iridium thin films on silicon with particular emphasis on the epitaxial growth of IrSi3 on (111) and (001)Si. The formation of precursor phases, IrSi and IrSi1.75, were found to occur mainly in the temperature ranges of 300-500 and 600-900°C, respectively. The stable IrSi3 was found to grow epitaxially on (111) and (001)Si annealed at 1000-1100°C. There are three dominant modes of epitaxial IrSi3 on (111)Si and one dominant mode of epitaxial IrSi3 on (001)Si. Interface structures of these different epitaxial modes were determined by diffraction contrast analysis. The quality of IrSi3 epitaxy in terms of the fraction of the silicon surface coverage, size, and the regularity of the interfacial dislocations was found to be the best in (111) samples annealed at 1000°C. Epitaxial regions of IrSi3, as large as 40 μm in size, were observed on (111)Si. No direct correlation between lattice match and quality of epitaxy could be found. The Burgers vectors of edge-type dislocations for several modes of silicide epitaxy were found to be along the directions with larger lattice mismatches in agreement with a theory by Markov and Milchev [Surf. Sci. 136, 519 (1984)]. A superlattice structure of IrSi3 was found from diffraction pattern analysis. The superlattice structure was determined to be of hexagonal structure and has a unit cell with dimensions three times larger than that reported for IrSi3.