Formations of TiSi2 thin films by a solid state reaction between Ti thin films and Si substrates and the effects of the conditions of Si substrates have been investigated. Low-resistant C54-TiSi2 films were formed by rapid thermal processes at 75$0^{circ}C$ on the undoped Si(100) substrate and at 80$0^{circ}C$ on the As or B-doped si (100) substrates as well as on As or B-doped poly-Si substrates. Cross-sectional TEM analyses confirmed the formation of small-grained C49 TiSi2 films by rapid thermal processes at $700^{circ}C$ on pre-amorphized poly-Si substrate by As implantation. The temperatures of the transformation to the C54 phase decreased in small-grained C49-TiSi2 films