Abstract: Based on the dynamical equations and the initial conditions, the oscillation of a simple pendulum with large amplitude is analyzed approximately. A method is introduced to estimate the period of a simple pendulum with an extremely large amplitude whose value is approaching π. The results show that the deviation between the ex- act value and estimated value of the period increases with the decrease of the oscillation amplitude. The linear ap- proximation leads to the increment of the errors.

Key words: simple pendulum, large amplitude oscillation, period