This paper will present a developed model to predict cutting forces in flat helical end milling based on a variable flow stress machining theory.  A chip geometry model is divided into a number of segments by discretizing the radial depth of cut. The infinitesimal chip geometrical section is considered to be an independent section for analysis. The maximum and minimum chip thicknesses are calculated for each chip segment and the average chip thickness is obtained to compute the cutting results such as forces, etc. The forces are summed for each segment to obtain the total forces acting on the system of the workpiece and the tool. Hence, the chip geometry for each segment is considered to be constant. The variable flow stress machining theory is employed to predict cutting forces in helical end milling process for these segments. Oblique cutting conditions are applied to cater for the chip flow direction due to the helix angle. The cutting forces can be predicted from input data of work material properties, cutter configuration taking into account the cutting conditions. The work material properties are represented by two material constitutive models - the constitutive equation which is modeled by the empirical power law stress-strain relation used by Oxley and his co-workers and a Johnson-Cook constitutive material model. The validation of the proposed model is achieved by correlating the experimental results with the predicted results. The correlation obtained is very encouraging.