An LED lamp has two preheating current control units respectively connected to a first electrode terminal and a second electrode terminal of the LED lamp, two lamp-side rectification circuits, and an LED light string. Each preheating current control unit has a ballast-side rectification circuit connected to one of the first and second electrode terminals and a load-varying circuit having a resistive load and a control circuit. The resistive load is serially connected to a DC output terminal of the ballast-side rectification circuit through the control circuit. The control circuit adjusts a resistance of the resistive load. Two input terminals of each lamp-side rectification circuit are respectively connected to one of the electrodes of each of the first electrode terminal and the second electrode terminal. The LED light string has two ends respectively connected to the two output terminals of each lamp-side rectification circuit, and has multiple series-connected LED elements.