In this paper, ductile irons with different microstructures were machined in a group of experiments using standard testing procedure and the effect of microstructure on the machinability was investigated. Bars (diameter 90 mm x 320 mm) of ferritic/pearlitic, tempered martensitic, and lower ausferritic microstructures were produced by various heat treatments and then tested to evaluate their machinability. Five groups of ductile iron specimens with various microstructures were tested following ISO 3685: 1993 (E) standard. Specimens were subjected to dry cutting; cutting force, flank wear width, and surface roughness were measured to compare the machining performances. In addition, further turning tests were conducted on the specimens of austempered microstructure at various feed rates (while keeping the cutting speed and depth of cut constant), and both surface roughness and cutting force values were recorded. The influence of feed rate on the machinability of austempered ductile iron was investigated and a correlation between feed rate and machinability, in terms of cutting force and surface roughness, was established. Not only the tangential cutting force component but also the feed force component was taken into consideration in these tests.