Based on the experiments about the Brazil twin appearing on -x plane {1120} of synthetic quartz, we have studied the formation mechanism of twinning by means of X-ray topography and electron probe micro-analysis and discussed the disadvantage in Kern's theory of nueleation dynemics of twinning. The new concept of strain barrier in twin boundary (SBTB) is proposed and it is considered that the crystal imperfection raises the energy state of the crystal itself and results in the decrease of height of strain barrier and easy formation of twin. We have also discussed the correlation of twinning with seed orientation, impurities and crystal defects in the light of crystal growth experiments and testing of crystal defects, and emphasized the influence of defects on twinning. Finally, the development of twin has been analysed from the view point of re-entrant corner growth mechanism.