Manganese silicides are promising candidates for microelectronics and spintronics materials. A good understanding of their growth mechanisms is a crucial step toward their practical applications. In this paper, a Mn film of ～4 monolayer is deposited on a Si(100)-21 surface by molecular beam epitaxy. The solid reaction between the Mn film and the silicon substrate in a temperature range of 250750℃ is studied using scanning tunneling microscopy. At room temperature, the as-deposited Mn atoms do not react with the silicon atoms and the film consists of disordered Mn clusters. When the sample is annealed at a higher temperature than 290℃, the Mn begins to react with the Si and forms small three-dimensional (3D) islands of Mn-rich silicides and silicide islands of dendritic shapes. When the annealing temperature reaches 325℃, small tabular islands, which correspond to MnSi, start to grow on the Si substrate. At an annealing temperature of 525℃, silicide islands with dendritic shapes all disappear; meantime several large tabular islands, which correspond to MnSi1.7, are formed. When the annealing temperature is higher than 600℃, 3D islands and small tabular islands all disappear while large tabular islands remain there. These results demonstrate that the morphology and the structure of the film strongly depend on annealing temperature. The average size (area) of the remaining islands increases with the increase of annealing time. Time dependence of the averaged island area indicates that the growth of the islands follows the diffusion limited Ostwald ripening mechanism.