There are some shortages to ascertain attribute weight based on rough set in current studies. In this paper, attrib- ute importance represented by rough set is studied deeply. Aiming at the existing problems, algebra presentation of rough sets is proved to be more comprehensive than its information presentation, then a new method of ascertaining attribute weigh is put forward based on rough set conditional entropy. Finally, it is shown that the new method is more reasonable than the old one by an example.