The data composed of firms & time in a certain period of time are called “panel data” .This paper investigates changes in earning quality of the firms listed in Tehran Stock Exchange. The sample includes 123 firms of various industries listed in Tehran Stock Exchange from 2001 to 2010. The paper examines the earning quality of the firms from the view points of “accrual quality” “earning persistency” “earning predictability” and “earning smoothness”. The results indicate any qualitative characteristic of earning, except earning smoothness variable has a significant relation with the dependent variable.