The molecular dynamics simulations have been performed on microstructure and tra n sfer mechanism of liquid Al. The pair distributed function has been attained at different temperatures and cooling rates by means of tight－binding potential. T he proportions of small atom groups of different types have been analyzed by ado pting bond pair index method. The result indicates that if the cooling rate is l ow　 crystals will be formed　otherwise　amorphism will occur under relatively hi gh cooling rate. When the liquid Al forms crystal　the bond pairs 1421　1422 play an important role　 while the pairs 1551　1541 have the close connection with the formation of amorphism.