We studied the formation of first InSb molecular layer on the Si(111) substrate by depositing about 1 ml of Sb on In(4 x 1) reconstruction in the temperature range 170-350 degreesC, using reflection high-energy electron diffraction (RHEED) and auger electron spectroscopy (AES). The element Sb deposition at 210 degreesC resulted in the formation of an epitaxial InSb molecular layer (not psudomorphic). This surface showed a weak I x 1 RHEED pattern with a 'dramatic rotation' of the surface layer with respect to the Si substrate. Ar this temperature, the deposited Sb atoms do not replace the Si-In bonds. However, Sb deposition above 250 degreesC resulted in the replacement of In-layer by Sb atoms. Subsequently Sb formed various well-ordered reconstructions. which are highly sensitive to the deposition conditions. 1 ml Sb deposition on In(4 x l)reconstruction at 260, 300 and 350 degreesC formed 2 x 1, 2 x 2 and root3 x root3 structures, respectively. All these surface phases produced very sharp RHEED patterns indicating the long-range order. For Sb depositions above 300 degreesC, AES results showed negligible In presence on the Si surface, suggesting the coalescence of In reconstruction by the adsorbed Sb. This replacement reaction drastically changes the bonding nature of the Si surface, which explains the role played by the In(4 x 1)reconstruction for the InSb growth on Si(111) substrate.