Micro milling is one of the critical machining processes that is widely used and has the advantage of creating complex geometry in a wide range of materials. However, premature wear and breakage of the micro tools as well as the stability of the system become one of the challenges in micro milling. So, accurate prediction of cutting forces is needed for optimization and planning of the process. This study aims to develop a mechanistic model for the prediction of cutting forces in the micro end milling process from basic metal cutting parameters estimated from orthogonal cutting data. Instantaneous uncut chip thickness is calculated using an algorithm based on the exact trochoidal path of the tool tip considering tool run-out effect, minimum chip thickness, and elastic recovery of materials. The cutting force coefficients are estimated using a fundamental oblique cutting approach taking into account material strengthening effect and edge radius. To validate the model, cutting forces in the micro slot end milling process are simulated for mild steel using the developed mechanistic model and compared to the experimentally measured signals from literature. The results of cutting forces amplitudes comparison show an average absolute error of 15.36% for feed force and 12,87% for lateral force.