Dynamics of a rotating cantilever beam near its critical angular speed is investigated in this paper. The external, force is idealized as a periodic function which has the same period as the rotati ng frequency of the beam. The equations of motion are derived and transformed into a dimensionless form. A prescribed spin-up motion is employed for the rotating motion. Numerical study shows that the steady state and the transient responses of the beam are affected by the spin-up time constant and there exists a time constant at which the maximum transient response becomes minimum.