A constant current regulator includes a current sensing circuit loop connected to a switch unit of the constant current regulator to detect a current flowing through the switch unit and to generate a detection current that is in proportion to the current flowing through the switch unit. The detection current flows through a detection resistor to induce a detection voltage. A differential amplifier bases on a set voltage and the received detection voltage to generate an error voltage to a pulse width modulation controller, which in turn causes a gate driver circuit to control the switching operation of the switch unit thereby supplying a constant current to a load connected to an output voltage of the regulator.