Effects of boron addition on microstructure and mechanical properties of thin ductile cast iron have been investigated. The amounts of boron addition and the thickness of specimen have been varied from 0 to 0.4wt% and ø13㎜, ø10㎜ and ø6㎜, respectively. As the casting thickness decreased, the average diameter of spheroidal graphite was decreased and hardness of the cast iron increased. By adding boron, tensile strength increased and had maximum which is about 30∼40% higher than ordinary ductile cast iron with 0.17wt%B addition. With boron adding more than 0.2wt%, ferrite and pearlite gradually transformed to boron carbide, resulting in decrease of tensile strength. The elongation decreased continuously with boron content. The addition of boron about 0.002∼0.4wt% improves the wear resistance and tensile strength of thin ductile cast iron.