Ultrathin manganese films were deposited onto Si(111)-7×7 surfaces by electron-beam evaporation. The solid phase reaction between the manganese thin film and the Si(111) substrate between 300 and 650 ℃ was studied in situ by ultrahigh vacuum scanning tunneling microscopy (STM). The deposited Mn atoms form an ordered nanocluster array on the Si(111) surface at room temperature. When the sample was annealed at 300 ℃, the Mn nanoclusters increased in size and the nanocluster array became disordered. When the annealing temperature reached about 400 ℃ the Mn began to react with the Si and the products consisted of three-dimensional (3D) and tabular islands. The tabular islands are a MnSi compound and the 3D islands are Mn-rich silicides. The MnSi tabular islands were the only product when the sample was annealed at 500 ℃. At an annealing temperature of 650 ℃, the MnSi tabular islands converted into large 3D islands which were likely Si-rich manganese silicides and the destroyed substrate surface reverted to the 7×7 structure through recrystallization.