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Microstructures of 0.08% C-O.09Ti% and 0.08% C-O.025Nb% -0.09Ti% high strength heavy plates produced by TMCP are composed by banitic ferrite plates and martensite-austenite ( MA) constitutes locating at plates boundaries.After tempering process,MA decomposed and carbides precipitated as well as boundaries of bainitic ferrite plates became weaken.The amount of MA was found increasing in Nb adding steels.TiCN、 Ti4C2S2 were found in Ti microalloying steels.Coarsen Nb-Ti complex precipitations (1~2μm) were found in Nb adding steel.The yield strength,tensile strength and Charpy impact energy decrease with Nb addition at TMCP processes.However,the difference of mechanical properties of Nb-Ti and Ti microalloying steel plates is small after tempering process.Steel plate with optimized strength and toughness was obtain by finish cooling at 500℃ following by temper at 550℃.Solution Nb in austenite is 0.01% at experimental austenitize temperature.It is reasonably that the mechanical properties decreasing of Nb-Ti steel relate to the pinning effect of coarsen Nb-Ti precipitates to mobile dislocation and boundaries is lower than that of the fine precipitates in Ti microallying steel.