The cemented carbide YG30 and steel 1045 were welded with Co-Fe-C filling alloys with different carbon contents by the tungsten-inert-gas (TIG) arc welding. η phase formation at the welding joints was investigated through scanning electronic microscopy (SEM). The results show that the average composition of η phase is W-25, Fe-22, Co-19, C-24 (mass, %), which is a kind of carbide enriched by Fe, W，and Co. The amount of η phase formed near the interface of YG30 and weld bead is related to the C content in the filling alloy. Namely the amount of η phase decreases with the increasing of the C content in the filling metal. When the C content reaches to 0.8 wt%, no η phase forms. The reason of which is that the added C reduces and/or restrains the resolving of the WC that locates at the interface, so that inhibit the W and C to form η phase with Fe and Co. The existence of large-size η phase near the interface is mainly attributed to the aggregation of small size η phase with the unsolved WC due to the stir of liquid metal, and then growing up.