The progress in femtosecond laser has enabled us to observe coherent motions of lattice and molecular vibrations in solids. Nevertheless the study of coherent phonons in Si, which is an important material for modern devise application, has not been made so far. We report the dynamics of transient interaction of electrons with phonons, followed by generation of coherent optical phonon, in which the pump-probe technique with 400 nm, 10 fs laser is used for the observation. A comparison of the results is made with other semiconductors.