In this paper, a TRIAC dimmable controller for quasi-resonant flyback LED driver is realized. Nowadays, many countries take effort to save energy. Because lighting system takes part of approximately 20% of world energy consumption, by replacing the traditional incandescent bulbs with light-emitting-diode (LEDs), electrical energy can be saved significantly. TRIAC dimmer is commonly used for traditional incandescent bulbs in Europe and America countries. The TRIAC needs a holding current to keep it conducting. The TRIAC will misfire and cause the LED light output flickering when the holding current is insufficient, so that to solve this problem, this thesis proposes a method to estimate the holding current requirement for various TRIACs by detecting the misfiring phenomenon. Then the controller will automatically adjust the bleeder current to meet the TRIAC's holding current. Compare to the traditional constant current bleeder circuit, the proposed method can reduce the power consumption on the bleeder circuit significantly. Furthermore, this controller utilizes primary-side quasi-resonant control which can reduce switching loss and improve the efficiency. Finally, this controller is fabricated with TSMC 0.25 μm 60 V CMOS high voltage mixed signal general purpose process and applied to an input voltage of 90∼264 Vrms, output voltage of nominal 40 V, and constant output current of 600 mA/24W hardware prototype to verified the feasibility of the proposed control.