Abstract: In General Motor Co. of America, since 1956, bismuth and boron are used as alloying elements in mass production of malleable cast iron crankshaft. But current informations about the effect of bismuth on cast iron are very few. This paper gives some data about the effect of bismuth on graphitization of cast iron during solidification.1. The carbon equivalent (C+1/3 Si and C+2.861og Si) are plotted against the quantity of bismuth in cast iron (residual) and two structural diagrams are shown.2. Even a little content of bismuth on cast iron (for instance, 0.009% Bi for liquid iron containing 2.63% C & 1.48% Si) is enough to change the grey iron into white.3. Provided that the carbon equivalent of cast iron is higher than a definite value, bismuth can not convert the grey iron into white at all. This critical carbon equivalent varies with the thickness of castings (cooling rate). For instance, the critical value of C+ V8 Si is equal to 3.10 for 40 mm diameter casting sample, while for the sample of d=16 mm this limit is over 4.30.When the value of C+1/3 Si is greater than 3.64, the 40 mm round castings will invariably solidify into grey no matter how much bismuth is added into cast iron.4. Effect of bismuth on graphitization may be concluded as follows: Bismuth is a very strong carbide stabilizer during solidification of cast iron, while during eutectoid transformation of cast iron bismuth seems to be a weal graphitizer. 5. When bismuth is added in ladle, only a little quantity of bismuth can be absorbed by ,cast iron. In our experiments, the percentages of residue bismuth in cast iron were never higher than 0.018%.