Tool flank temperature at various intervals after cutting in dry turn-milling of AISI 1045 steel is measured using a two-color pyrometer with an optical fiber. Complicated undeformed chip geometry, which depends on cutting tool diameter, nose radius, number of tooth, workpiece diameter, tool-work revolution speed ratio, depth of cut, feed per tooth, tool axis offset and cutting distance, is analyzed and visualized by the 3D-CAD system. The effect of cutting parameters associated with material removal rate MRR such as workpiece diameter, workpiece revolution speed and feed rate on tool flank temperature is investigated in this paper. Workpiece diameter affects tool flank temperature, and 10 mm larger diameter causes approximately 40 °C higher temperature in any workpiece revolution speed due to the variation of undeformed chip geometry analyzed by 3D-CAD. Tool flank temperature increases with feed rate and workpiece revolution speed because the cross-sectional cutting area of undeformed chip increases with workpiece revolution speed, and cutting time during the engagement of each flute also increases with feed rate. Almost same values are obtained between the tool flank temperature and the material removal rate MRR when both workpiece revolution speed and feed rate are changed.