When a body rolls down without soliiping along an inclined plane of inclination θ , it rotates about a horizontal axis through its centre of mass and also its centre of mass moves. Therefore, rolling motion may be regarded as a rotational motion about an axis through its centre of mass plus a translational motion of the centre of mass. As it rolls down, it suffers lossin gravitational potential energy provided translational energy due to frictional force is converted into rotational energy.

Note : In fact, friction is needed to cause the body to roll. However the rolling friction is so small that we can use conservation of mechanical energy.