In ultraprecision machining for microchannels, deformation occurs with high frequency because the diamond tool pushes against the channel structure during the machining process. Conservative cutting conditions decrease the cutting force and avoid deformation, but they reduce productivity. Therefore, it is important to select suitable cutting conditions in practice. This article presents a theoretical decision model for optimum machining conditions in manufacturing rectangular micropatterns. The model involves the prediction of cutting force and deformation. To forecast deformation phenomena, a rectangular pattern was considered as a cantilever beam with a distributed load, and the maximum principal stress that acts on the rectangular micropatterns could then be determined. For verifying this solution, several experiments were carried out to obtain a rectangular pattern by single-point diamond turning. Finally, the machining condition decision model was implemented and verified in practice.