Mn-implanted Si with postannealing treatment at 873, 973, 1073, and 1173 K exhibited ferromagnetism at room temperature. Ferromagnetism was enhanced as the annealing temperature increased. The largest value of saturation magnetization was found in the sample annealed at 1173 K. X-ray absorption near-edge structure spectra showed no formation of substitutional-interstitial Mn-Mn dimmers in all samples. On the other hand, high-resolution transmission electron microscopy and glancing incidence X-ray diffraction revealed a variety of MnSi compounds. We propose that the ferromagnetism has the origin from the MnSi1.7 nanoparticles as well as the MnSi 1:1 phase, which possesses strong short-range spin correlation. The formation of antiferromagnetic Mn5Si3 compound in 873 and 973 K annealed samples substantially reduced the magnetism. This paper provides new insights into the understanding of ferromagnetism in Mn-implanted Si diluted magnetic semiconductors.