An understanding of the plastic deformation behavior of Ti6Al4V (Ti64) is of great interest because it is used in aerospace applications due to its high specific strength. In addition, Ti alloys have limited slip systems due to hexagonal crystal structure; hence twinning plays an important role in plastic deformation. The present work focuses upon the grain size effect on plastic deformation behaviour of Ti64. Various microstructures with different grain size were developed via annealing of Ti64 alloy in α-β phase regime (825°C and 850°C) for 4 hours followed by air cooling. The deformation behavior of these samples was investigated at various deformation temperature and strain rate conditions. Detailed microstructure studies showed that (i) smaller grains undergoes twinning only at low temperature and high strain rate, (ii) large grain samples undergo twinning at all temperatures & strain rates, though the extent of twinning varied. © (2015) Trans Tech Publications, Switzerland.