Abstract: Laser-arc hybrid welding process is closely related to the arc atmosphere.By using a laser power meter, a beam quality diagnosis instrument and a high speed camera, the beam and arc characteristics were investigated during the vertical interaction between a CO₂ laser beam and a TIG arc in argon and helium atmosphere comparatively.The experimental results demonstrate that the laser power attenuates dramatically, the beam defocuses and the beam quality is seriously worsened while the arc voltage drops, the arc column expands, and even combustion wave supported by a laser generates in argon atmosphere.However, the beam and arc characteristics seldom change during the laser-arc interaction in helium atmosphere.The difference of the laser-arc interaction results from the great difference of the electrons number density due to the difference of the gas ionization energy.The electrons number density in the helium arc is 10 times less than that in the argon arc, and thus the inverse bremsstrahlung absorption coefficient of the helium arc is two orders of magnitude lower than that of the argon arc.Meanwhile, there is less difference of the refractive index between the helium arc and the air, so there is no obvious refraction of the helium arc to the laser beam.