This paper presents a duty cycle regulation on class L-Piezo inverter to reduce the switching losses in GaN HEMTs. The class L-Piezo inverter is a variant of the class φ2 where the filtering of the second harmonic is insured by a piezoelectric resonator. A ZVS detector senses the error compare to the ZVS operation and a regulation loop optimizes the duty cycle to reduce that error and maintain the ZVS operation on the switch whatever the operating conditions. Simulation are carried out with variable input voltage and variable output load to optimize the regulation. Finally, a 10.4MHz class L-Piezo prototype validates experimentally the regulation loop. The experimental inverter delivers up to 30W with an efficiency of 88.2% with the duty cycle regulation.