In the CO2 gas shielded arc welding, spatters are scattered around and bonded to the surface of base metal, and it is still unclear what are the main factors of the difference of their bonding force. In this paper we examined the bonding force of spatters, which were bonded on the surface of base metal (SS400) in different conditions and temperatures in the CO2 gas shielded arc welding (using the solid wire of 1.2 mm in diameter). The conditions of the surface of base metal were as follows: (1) The surface has scale, (2) The scale of the surface was removed with the surface grinder (Rmax=0.6 μm), (3) The ground surface was fumed, and (4) The scale of the surface was removed with the disc grinder (Rmax=7∼16 μm). The temperature of base metal was raised by the ceramic heating unit of 14 mm in diameter, which was set up at the back of base metal. The bonding force was measured through the shear force of the spatter. 400 spatters were examined on each surface. Most of the dimeters of the spatters were 0.4 to 1.4 mm. When the temperature of base metal was below about 450 K, spatters on the surface with scale was not hot enough to melt the base metal and could not bond there by melting the surface of base metal. Most of their shear stresses were below about 40 MPa, but as the temperature raised, the shear stress was increased up to nearly (300) MPa. The fume on the surface or the roughness of the surface did not affect the bonding force as directly as the scale on the surface. The temperature, however, affected the bonding force. When the temperature of base metal was below 450 K, the bonding force became a little smaller. When the spatter bonded on the surface, because of the heat transfer, the change of the microstructure and correspondingly hardening was found to occur. When the spatter and the heat-affected zones of base metal were examined, their Vickers hardnesses indicated a little higher value (some spatters had the value of 470(Hv)) than the hardness of base metal (156∼165 (Hv)), although it depended on the conditions of the surface and the temperature.