This paper presents a single-stage inverter with variable frequency modulation (VFM) based on LLC resonant converter. And LLC converter is a common topology of dc/dc conversion. LLC resonant converter can achieve high efficiency and soft-switching performance. Since the dc gain curve of the single-resonant LLC converter is flat when the switching frequency is larger than the resonant frequency, namely fs>fr, an additional L-C series resonant branch is paralleled to the original resonant tank to introduce higher-order-harmonic resonant current and a zero-gain point to the gain curve. Higher-order-harmonics help to deliver power and the zero-gain point enlarges the gain range which improves output THD and reduces the switching frequency range. A 1.2 kW prototype is built to demonstrate the performance of the proposed inverter. Zero-voltage-switching (ZVS) and zero-current-switching (ZCS) are achieved on the primary side and secondary side, respectively. And 97.3% efficiency and 2.17% voltage THD are achieved at full load condition, while 97.2% efficiency and 3.2% voltage THD are achieved at half load condition.