The mechanical properties of AISI 4140 steel were studied at different strain rates (0.167, 0.833, and 1.667 min^-1) for various microstructures. To investigate the influence of microstructure, spheroidizing, normalizing, and quench tempering (single tempered and double tempered) were used to obtain a range of microstructures and strength levels. To study the dependences of the mechanical characteristics of AISI 4140 steel on both the strain rate and the microstructure, tensile tests and microscopic evaluation were performed. The results showed that not only do the yield and the ultimate strength depend on both the microstructure and the strain rate but also so does the appearance of the yielding phenomenon. Also the results showed that an increase in strain rate causes the strain-hardening exponent n and the strength coefficient K to decrease in both quench-tempered and normalized structures but has a negative effect on the spheroidized structure. In fact, the sensitivity of the spheroidized structure to the strain rate is higher than those of the other structures. Microscopic evaluation proved that the Lüders strain width of the spheroidized microstructure is much lower, because of its soft ferrite matrix.