Self-assembled nanowire growth on semiconductor surface is based on deposing sub-monolayer material over the surface. Even though high resolution STM image gives plausible surface analysis, determination of the nanowire structure is the most difficult part of these experiments. Due to the this reason, first-principles investigation is essential to understand the one dimensional nanowire structure grown over the surface as well as the STM images of these structures. Recently, iridium silicide nanowire on Si (001) surface is observed. In this thesis, we study formation of the nanowire after deposition of Ir on Si(001). Ab-initio plane wave pseudopotential calculations are performed for number of iridium silicide nanowires generated by increasing iridium coverage on Si(001) surface. For the iridium coverage as 0.125 ML, the possible nanowire formation is analyzed and its calculated STM images are compared with experimental STM image. As a result of our detailed analysis, we suggest that the STM image observed at experiment doesn’t consist of Ir atoms since Ir atom tends to be buried into the Si bulk. We model the possible nanowire formation which is consistent with pseudo-STM calculation. According to our model, iridium is placed at the troughs between the dimer rows on the surface and it creates a structure by breaking the Si dimer bonds. The coverage implied by the model, is consistent with experimental numbers.