Experimental study was carried out with 38.6kHz High frequency (HF) current to join the Q345 steel with thickness of 5.5mm. Stable fully penetrated keyhole mode welding process was successfully achieved. Compared with ordinary K-TIG, the threshold current to create stable fully penetrated keyhole decreased from 430A to 340A in HF K-TIG at the welding speed of 390 mm/min. The welding current window for stable keyhole welding process expanded from 430A∼460A in ordinary K-TIG to 340A∼420A in HF K-TIG. Grains in HF K-TIG weld melting zone were much refined, and the amount of acicular ferrite increased. Microhardness curve of HF K-TIG weld was increased overall compared to that in ordinary K-TIG one, the maximum value was increased from 199HV to 212HV. The tensile curves reflected that the strength of the HF K-TIG weld is comparable to that of base metal. The results suggest that HF current is promising to improve the keyhole stability and refine the weld microstructure in the K-TIG welding.