The white layer formed in machining has significant impacts on the friction property, fatigue resistance, and service life of products. This paper presents an analytical model for white layer prediction in orthogonal cutting based on phase transformation mechanism. The effects of stress, elastic, and plastic strain on phase transformation temperature are taken into consideration. A function related to cutting temperature and phase transformation temperature is defined to determine the white layer thickness. The theoretical model is validated by machining AerMet100 steel under different cutting conditions. Optical microscope and X-ray diffraction (XRD) are employed to analyze the microstructures of the white layer. A phase transformation is detected in the white layer region, and the predicted white layer thicknesses are in good agreement with the measured values. In addition, the plastic strain is found to be the major factor that causes a reduction in phase transformation temperature. This work can be further applied to optimize cutting conditions to improve machined surface integrity.