This study was conducted to evaluate the effect of the grain size and orientation on the formation of deformation twins in Ti-15Mo alloy. For this material, cold rolling was carried out to achieve a 90% thickness reduction on the samples. In order to recrystallize the severely deformed alloy and to obtain two types of grain sizes, the alloys were then annealed at 900 degrees C for 1 and 30 min, respectively. After this work, electron backscattered diffraction was introduced to analyze grain boundary character distributions. As a result, the coarsened grains (average size similar to 154 mu m) formed denser deformation twins relative to that of the finer grains (average size similar to 25 mu m), which was attributed to the grain boundary constraint and orientation. These effects led to an enhancement of the strain-hardening constant of the alloy, which in turn resulted in a notable increase in the elongation without any significant decrease in the tensile strength. In this study, we systematically discussed the formation of twins in terms of grain size and Schmid factor.