This paper describes a new biomachining method by using microbially influenced corrosion. Differences between mild steel and copper in the metal biomachining through Thiobacillus ferrooxidans of a kind iron oxidizing bacteria have been investigated in three types of environment of 9K medium, the bacteria-cultured solution, and the cultured and sterilized solution. Losses in mass of SS400 by biomachining in 9K medium with the cultured solution were larger than those of copper. Losses in mass of SS400 and copper by biomachining in the cultured solution were respectively about 30 mm/y and 6 mm/y as the dissolved amount was converted into thickness. The surface of SS400 by biomachining was in rough compared with copper's in 9K medium and the cultured solution, that is the surface roughness of SS400 was abut 30μm whilst one of copper's was abut 3 μm.