An LED driving device includes: a rectifying circuit for outputting a DC voltage to a string of M LED units; (M−1) first switching circuits each coupled between a corresponding one of first to (M−1)th LED units and ground; and a second switching circuit coupled between an Mth LED unit and ground. When the DC voltage is sufficient to turn on first to kth LED units, where 1≦k≦M, the kth LED unit is coupled to ground through first and second conductive paths provided by a resistor unit, and a corresponding first switching circuit or the second switching circuit, and each of the first to (k−1)th LED units is coupled to ground through a third conductive path provided by a corresponding first switching circuit and the resistor unit.