We investigated the C49-TiSi2 epitaxial orientation dependence of the C49-to-C54 phase transformation rate for samples with different pre-amorphizing As implantation (PAI) conditions. The C49 epitaxial orientation to the Si(001) substrate is characterized on the basis of the (131)rocking curves obtained from grazing-incidence X-ray diffraction (GIXD) measurements. We found that the phase transformation rate increases with decreasing C49 epitaxial orientation, and this dependence is very sensitive for the samples with PAI, For comparison with the orientation dependence, the C49 grain size dependence was also examined. The C49 grain size distributions were obtained from transmission electron microscope (TEM) images. Although the phase transformation rate seems to roughly increase with increasing C49 grain size, it does not show a clear relationship. Therefore, we conclude that the C49-to-C54 phase transformation rate is largely determined by the C49 epitaxial orientation, although the effects of C49 grain size and the density of As-ions cannot be ignored for the PAI process.