The direct numerical simulations of turbulent flow in a rotating channel are executed, with the attention on a transient state after an onset of the rotation. Some different rotation numbers which contain 0, 0.5, 1, 2.5 and 5 are tested. To clarify a relationship between rotation numbers and the alteration of turbulence structures, we paid notice to temporal evolutions of wall friction velocities on both sides, volume-averaged velocity, kinetic energy and its production and dissipation. We found that the time evolutions of flow rates indicate non-monotonic behaviors due to the system rotation. The major reason of it is the difference in response of wall friction velocities on both walls. Moreover, we applied an analysis based on the theory of the invariants of Reynolds stress tensor to associate the status of turbulence with turbulence structures. From the result of the analysis, the three-dimensionality of turbulence is emphasized by the system rotation except for the vicinity of the wall on the pressure side. On the suction side, however, flow field indicates the feature of two-dimensionality.