Abstract: With the equivalent length of pendulum，considering the effect of pendulum size on its period，the variation of period for leaky circular truncated cone pendulum with the height of liquid in it is deduced. Consequently， with the height of liquid descending，the period of the leaky pendulum increases first，reaches its maximum and then decreases. Specifically，the greater the density of the fluid is，the greater the maximum is. The above conclusion can be used to other rotating bodies，such as cone and cylinder，etc. After that，using the series method，it is found that the period of leaky pendulum is mainly determined by its nature，that is，pendulum length and pendulum size. It means that the pendulum length is far larger than the pendulum bob，the variation of its mass can be ignored， which can be used to demonstrate the motion of simple pendulum. Finally，the period of leaky circular truncated cone pendulum varying with the liquid height has been measured experimentally，which is in good agreement with the present theoretical conclusion.

Key words: leaky pendulum, the equivalent pendulum length, period