AISI4340 hardened steel have a vast functionality in industries. Hard machining of this steel have several benefits such as, higher productivity, lower production cost and improved workpiece properties. In machining operation, ultimate surface roughness is the most important characteristic of machined surface and plays an important role in workpiece life. One of the effective factors on surface integrity is cutting fluid used in machining operation, which have health and environmental problems is spite of positive effects. As a result, using minimum quantity lubrication is considered as an alternative method. In present study, relations between milling parameters and final surface quality in milling of AISI4340 hardened steel, in the presence of lubrication systems including; dry, wet and minimum quantity lubrication have been investigated. Cutting speed, feed rate, axial and radial depth of cut have been considered as main parameters of milling operation. Totally, 90 experiments have been done using response surface method to analyze the effects of process parameters on surface roughness. Results revealed that feed rate and cutting speed have the most Influences on surface roughness. Also higher values of cutting speed and lower values of feed rate are necessary to reduce surface roughness. In addition, compared to other lubrication methods, minimum quantity lubrication have the best performance in surface quality, especially in high cutting speed and depth of cut.