Hard steels are widely used in automotive industry, molding and production of well drilling bits because of its high wear resistance and strength. The tendency to hard machine of these steels is growing in order to achieve high dimensional and geometrical accuracy, increased productivity and improved workpiece properties. In this research, relation between cutting parameters and final surface integrity in hard milling process of a workpiece made out of 4340 steel while using minimum quantity lubrication system is studied. Different parameters were considered in three levels as main milling parameters including: cutting speed, feed rate, axial and radial depth of cut and consequently the effect of these parameters on surface microhardness and white layer thickness were studied using Response Surface Methodology (RSM). The analysis of variance (ANOVA) showed that a model of quadratic polynomial function would work perfectly in order to estimate microhardness and it can also estimate experimental results while a linear model can evaluate white layer thickness changes, better. Also, Statistical analysis revealed that all cutting parameters increase microhardness and white layer thickness. Feed rate with 73.1% and cutting speed with 14.4% had more effect on microhardness comparatively. White layer thickness also varied between 7.6 μm to 16.1 μm while different cutting conditions were applied and cutting speed with 81.3% and feed rate with 9.4% had the most effects on white layer thickness.