Altering in surface forms cause to varying cutting forces in machining of inclined surfaces by ball end mills. The deflections appeared by cutting forces can result in form defects in workpiece. Finding deflection values and related form defects has a significant importance for keeping the production within the appropriate tolerances and in lessening production costs. The aim of this study is to evaluate form defects depending on tool deflection in milling of inclined convex and concave surfaces of EN X40CrMoV5-1/DIN 1.2344 using TiAlN-coated cutting tool. The experiments have been conducted with different cutting tools strategies and cutting parameters at related parameter levels. During machining real time cutting tool deflections are recorded by inductive sensors for the experiments. Then, the form defects of machined specimens caused by cutting tool deflections are determined by 3D optical scanning. The cutting tool deflection values and the values of 3D optical scanning form defects have been compared. The largest cutting tool deflections and form defects are detected at the ramping regions with the milling position angles of 45 degrees to 60 degrees for the convex and concave surface types. In addition, the regions that the largest tool deflections have been observed are the same regions with the largest form defects obtained by the optical scanning. As a result, at the determination of form defects caused by cutting tool deflection, the deflection values obtained by real time measurement are coincided with the measurement obtained from 3D optical scanning.