A daytime running lamp (DRL) controller for controlling power to a light source of a vehicle includes a switch, an engine starting-up detector and a switch control circuit. The switch is coupled to the light source. The engine starting-up detector is coupled to the switch, and generates a trigger signal if the engine is starting up. The switch control circuit is coupled to the switch and the engine starting-up detector, and includes a controller. The controller generates a first control signal to switch on the switch to power on the light source responsive to the trigger signal, and keeps outputting the first control signal if a holding signal is generated within a first time period after the trigger signal is received. The first time period is a time period during which the controller is reset after receiving the trigger signal.