Surface roughness and other surface characteristics of Al-Mg cast alloy (AC7A) were studied after machining by turning in low cutting speed ranging from 20 to 300m/min. Comparison was made of surface roughness obtained by using five different types of tools. Of these, carbide tool (K-type) and diamond compact tool resulted in favorable surface roughness. Consequently, experiments were carried out under different cutting conditions using only these two types of tool. As a result, it was found that favoravle surface roughness of 2.8 to 3.3μmRz and 1.3 to 2.3μmRz were obtained by using the carbide and diamond compact tools, respectively, when a cutting fluid was used with cutting depth of 0.1mm and feed of 0.045mm/rev. The surface machined with the carbide tool exhibited a dull luster compared with a glossier surface machined with the diamond tool. It was learned that the difference in the finished surfaces was due to a difference in the cutting mechanism, which is related to the presence and/or absence of built-up edges.