Industries follow a method to deliver their products to the market with the least cost. Stable development leads industries to overcome environmental problems and risks from human activities and so machining as one of the production factors follows these rules. Temperature is one of the main factors of wear and tool wear determines tool life, optimum speed and machining cost. Because of human and environmental risks using soapy water is gradually replaced by environmental compatible coolants. Cryogenic method that uses liquid nitrogen as tool cooler, is one of the important methods. In this research Ck45 and 100Cr6 steels have been used. The steel type of 100Cr6 is widely used for manufacturing of bearings being used in movable parts of machines. This steel is included in high-carbon, low-alloy steels and in ranking diagram is hypertectoid. This steel has 1.1 % carbon and is used when good wear properties or more strength than the strength of simple carbon steel is needed. Ck45 steel is a carbon steel which is included in group of heat treated steels. Moreover it is in group of high temperature resistant construction steels. This steel has much use in automobile parts and motors and is also used in high temperature manufacturing consumable screws (approximately 400° centigrade). Turning was performed with 2 cutting speeds 94.24 and 125.6 m/min with timings of 2, 4, 6, 8, 10 min and using dry, soapy water and cryogenic machining methods respectively and tool wear was measured in each case and tool life duration was evaluated. Taylor's equation constants related to cutting speed were calculated and used to find optimum speed of turning and machining cost was calculated and compared in 3 methods. In cutting speeds of 94.25 and 125.66 m/min for 100 Cr6 steel, using cryogenic coolant decreases wear ratio up to 48 and 55 percent in comparison with dry method, and also decreases 19 and 37 percent in comparison with soapy water method. Moreover results in cutting speeds of 94.25 and 125.66 m/min for Ck45 steel show 48 and 36 percent decrease in wear ratio in cryogenic method in comparison with dry method and 24 and 35 percent decrease in comparison with soapy water. Furthermore, cryogenic method leads to an increase of 30 and 50 percent tool life for 100Cr6 steel, 27 and 37 percent for Ck45 steel in comparison with soapy water, an increase of 82 and 120 percent for 100Cr6 steel, and the increase of 80 and 114 percent for Ck45 steel in comparison with dry method in 94.24 and 125.6 m/min respectively, that shows comparative advantage of cryogenic method in high speeds. Machining cost in cryogenic method can decrease up to 0.7 of the cost of dry machining and 0.8 of the cost of soapy water machining. With increasing the price of machining time and hardness of work piece, the cryogenic method cost has more decrease in comparison with the other two methods. The finish of final surface in cryogenic method is better than dry machining, however in soapy water at first roughness is better because of the property of lubrication of soapy water but after some time, because of more wear of tool, roughness is worse in comparison with cryogenic method. Because liquid nitrogen has little contact on final surface, cryogenic machining has no noticeable effect on structure and hardness of final surface. Key words Cryogenic machining, Ck45 steel, 100Cr6 steel, Tool life, Surface roughness, Machining cost