Tool wear is one of the most important aspects in metal cutting, especially when machining of hardened steels is concerned. It is an important process because all manufacturers are continually seeking ways to manufacture their parts with lower cost, higher quality, rapid setups, lower investment, and smaller tooling inventory while eliminating non-value added activities. Hardened steel AISI 4140 material is commonly used to produce automotive parts such as shafts, gears and bearings. Machining this material significantly increases the temperature and friction in the cutting zone. Liberation of heat and generation of friction associated with hard turning operation are problems which not only reduce the tool life but also impair quality of the product. Cutting fluids have been the conventional choice to curtail friction and temperatures in machining because of its lubrication and cooling actions. Using nano-cutting fluid instead of convectional cutting fluid is novel approach to solve cited problem. Nano-fluid is a colloidal mixture of nanometer-sized ( 100 nm) metallic and non-metallic particles in conventional cutting fluid. Addition of the nanoparticles can alter lubricating properties, and convective heat transfer coefficient (cooling properties) of nano-cutting fluids. In this research work, Nano-cutting fluid is made by adding (0.0-1.5) % Tio2 nanoparticles with average size of 20 nm to distilled cutting fluid. Cutting speed, feed rate, concentration of nano-cutting fluid has been considered as main parameters of hard turning operation. Totally, 45 experiments have been designed using full factorial method to analyze the effects of process parameters on maximum flank wear. These influences, moreover, are compared with the outputs of similar tests through dry machining. The obtained results showed 1.5% volume fraction of TiO 2 Nanoparticles added to water as cutting fluid was considerably reduced maximum tool flank wear, machining force and surface roughness in comparison to dry machining. The investigations indicated that TiO 2 Nano-fluid reduced maximum tool flank wear, surface roughness and machining force by 30%, 40% and 35% respectively. Moreover, the results illustrated that the lowest maximum flank wear obtained in cutting speed 61.23 m/min, feed rate 0.11 mm/rev and cutting nano-fluid containing 1.5% volume fraction of TiO 2 nanoparticles. Furthermore, maximum tool flank wear was predicted by thermo-mechanical analysis with Finite Element Method-based procedure. Deform-3D FEM software was used to simulate the cutting process and a suitable subroutine was implemented into the software in order to evaluate the maximum tool flank wear and to update the tool geometry. Finally, maximum tool flank wears were extracted from experimental tests and numerical predictions were compared. Keywords: Tool wear, Nano-Fluid, Finite Element Method, Hard Turning.