In this paper, a single-phase 1kW off-line inverter under critical conduction mode (CRM) with high switching frequency using 650V GaN devices is studied, designed, and tested. For CRM full-bridge inverters with unipolar modulation strategy, if the output voltage value of the inverter is less than a half of its input, the drain to source voltage of a GaN devices would only resonate to some valley point. So zero-voltage-switching (ZVS) cannot be achieved within the full output voltage range, meanwhile power loss is often generated on the instant of the following active switch. For the purpose of minimizing the switching loss, including basic CRM working principle, extra-time of SR switch for achieving ZVS in whole line cycle, programmed on-time control are given. The simulation of CRM full-bridge inverter based on unipolar modulation is built to verify the rationality of the design. Finally, a 1kW GaN off-line Inverter is built and demonstrated with the fixed ZVS time margin based on proposed strategy.