A combination of transmission electron microscopy, scanning electron microscopy and optical microscopy was used to study deformation and fracture behaviors of lamellar microstructure (LM) Ti-55531 alloy during tensile and torsion tests at room temperature. Results indicate that loading modes have a significant influence on deformation and fracture mechanisms of LM Ti-55531 alloy. First of all, the tensile stress-strain curve of LM Ti-55531 alloy is different from its torsion stress-strain curve. The stress could decline at the end stage of the tensile test, while it is not during the torsion experiment. Secondly, deformation mechanism of tensile test is a mix mode which combines dislocation slip, twinning and shear of secondary α_s phase, while deformation of torsion test are controlled predominantly by dislocation slips and shear. Thirdly, fractographs of tensile and torsion tested specimens seem to possess different morphologies. Fractographs of tensile specimens are cliffier than torsion specimens. The tensile samples show a ductile failure, including dimple, cleavage and inter-granular fracture mechanisms. The fracture of torsion specimen is still a mix mode type but with more shear dimples. At last but not the least, no matter samples under tensile or torsion loading, the failure of LM Ti-55531 alloy are controlled by the highest shear stress. And the shear stress has much more influence on the failure of LM Ti-55531 alloy than the rectangle stress.