This paper reports an investigation of material microstructure effects on tool wear in microscale machining of multiphase materials. A new generic approach is proposed to estimate the tool wear that utilizes empirical data about the effects of micromilling process on cutting edge radius. Experiments were conducted to study independently the influence of two main phases in steel, pearlite and ferrite, on tool wear under different cutting conditions. Based on this empirical data, two regression models were created to estimate the increase of cutting edge radius when machining single and multiphase steels. To validate the models they were applied to predict the tool wear when machining two different multiphase steel samples. The results showed a good agreement between the estimated and the actual tool wear.