The formation process of Si-coated C-60, C60Sin (n = 1-60), was theoretically investigated by examining both the geometric structure and standard heat of formation (Delta H-f(0)) for individual C60Sin clusters using a semiempirical molecular orbital method. For a small number of Si atoms (n < 3), it was found that each Si atom is preferentially bound to the 6/6 bonds (double bond between adjacent hexagon rings) of the C-60 substrate independently. On the other hand, for a large number of Si atoms (n greater than or equal to 3), besides the bond formation between Si atoms and the C-60, the adjacent Si atoms tend to form chemical bonds with each other. The n-dependence of Delta H-f(0) for the C60Sin was quantitatively interpreted by the number of Si atoms with dangling bonds in the framework of the isolated pentagon rule (IPR). It was revealed that the Si-Si bond formation is an essential process to successfully synthesize the C60Si60.