A power generator includes a top, a support, a bearing, a rotating device and a transforming device. The top has a body and a spindle penetrating the center of the body, wherein the spindle is substantially vertical to the ground. The support is positioned on the ground to support one end of the spindle. The bearing holds the spindle and is located apart from the support. The rotating device rotates the top. The transforming device transforms the rotational energy of the top into power.