This research aims to unravel the contribution of different dimensions and frequency bands of lexical knowledge to the ability of reading and writing by testing the vocabulary knowledge ，reading and writing ability of 136 first-year English majors． The vocabulary tests assessed three dimensions of word knowledge ，including word recognition ，spelling and collocation ，the first two dimensions covering 2000 ，4000  and 6000  three  frequency  bands  while  the  last  covering  only  2000  band． Correlation  and multiple regression analyses of the data show  that  collocation  knowledge  of  band 2000  was  most  closely  related  to  reading  profi- ciency ，able to account for 36． 0%  of difference ，followed by recognition knowledge of band 4000 ，the two variables together re- sponsible for 4 1． 3% of variance．  The effects of other frequency bands or aspects of lexical knowledge were not pronounced． Fur- thermore ，collocation knowledge of 2000  also made a unique contribution  above  all  other  variables to  the  quality  of  writing ，ex- plaining 24 ． 8% of variance． This  study  suggests  strengthening  students ’collocation  knowledge  of  high  frequency  words  to  a- chieve better command of  English language．