The application of austempered ductile cast iron which showed load capacity comparable to SCM430 for making spur gears, to worm gears was attempted in addition to spur gears. However, with the worm gear, unlike the spur gear, considerable heat is generated along the tooth line because of large sliding. Acctrdingly, if hard material dose not harmonize with both worm and worm wheel, remarkable wear of seizing occurs. Consequently, in the present study, ductile cast irons with various hardnesses were prepared, and worm gears were subsequently prepared. In test operations with these worm gears, wear of tooth, temperature of tooth surface and transmission efficiency were investigated and compared with those of AlBC and S45C worm gears, which have been used formerly and the practical applicability was examined.