In this article, three-dimensional micro-end-milling finite element models were built to analyze the mechanics behavior and wear of microtool while machining aluminum alloy (Al 2024-T6). The cutting force, equivalent stress, and wear characteristics of microend cutter were dynamically simulated to investigate the tool runout effects. The simulation results show that the cutting forces in micromilling are very small as compared to conventional scale cutting. The maximal equivalent stress located on the tip of cutting edge, which produces distinct size effect on tool wear. Tool runout is found to have a significant influence on the mechanics behaviors and wear of microend cutter. Experimental verification of the simulation model was carried out by micromilling experiments. It is shown that the wear of microend cutter mainly occurs in tool nose. The small tool runout causes considerable rapid tool wear and breakage while deflection plane is parallel to the cutting edges of tool (gamma=0 degrees). In order to minimize the equivalent stress and tool wear, it is recommended that the axis of microend cutter should be consistent with the spindle axis and the deflection plane is perpendicular to the cutting edges (gamma=90 degrees).