Abstract: Experiments have been made on microalloyed medium-carbon L45 steel under the condition of controlled rolling and controlled cooling. The various mechanical properties and volume fraction of pre-eutectoid ferrite f_a, size of ferrite d_a, free distance of ferrite λ_α and and the interlamellar spacing of pearlite S₀ have measured. By using multiple regressive analyses to research, as a result, the mathematical expressions of relation between the various mechanical properties and structural parameters of the controlled rolled and controlled cooled microalloyed medium-carbon steels have been obtained. According to the informations provided by these mathematical expressions, into the Strenthen-toughening mechanizm of the controlled rolled and controlled cooled microalloyed mediumcarbon steels have discussed.