Abstract: The relative motion equation of Foucault pendulum is solved by using the conservation law and its exact solution is a hypotrochoid in the polar coordinate. Both curvature radius and periodic formation of trajectory are determined by means of the geometric property of hypotrochoid. It is proved that the rotation speed of vector of velocity is always equal to -ωsin λ.

Key words: Foucauh pendulum, rotation of oscillation plane, curvature radius, hypotrochoid