Abstract: The relationship between the manganese and niobium contents, the proce ssing parameters （heating temperature, rolling schedual, cooling and aging after rolling）, the microstructures, the mechanical properties and the fract-uve behalior of 06MnNb steels were investigated.By employing the procedure of lower heating temperature. optimizing rolling schedual and interrupted cooling with a cooling rate of 17℃ per second between 780°-600℃ a complex microstructure containing fine acicular ferrite and small amount of fine M/A inlands and granular bainite can be obtained.This complex microstructure is superior to ferrite pearlite structure is both yield strength and low temperatare toughness. The main factor defermining streugth and toughness-of steol is the mean diameter of acicular ferrite.The toughness of bainite dependsopon its parketsije.The amount and parket sije of gaunlar banite is affected by heating temperatare and amount of deetormation.