In this research, the reliability qualitative characteristic as a measure for assessing the earning quality of listed Tehran Stock Exchange companies has been studied. To study the subject, Three hypotheses were formulated which showed the market's response to the reliability qualitative characteristic and measures such as accruals quality, conservatism and Working capital accruals abnormal were used for operating the reliability qualitative characteristic. In the first hypothesis, Earning Response Coefficient (ERC) and two hypothesis R2 (the explanatory power of earning) were compared in two portfolios formed based on the reliability qualitative characteristic and in the three hypotheses, the relationship between the cost of capital and the reliability qualitative characteristic was studied. In order to do the studies, 90 listed Tehran Stock Exchange companies during the 1998-2007 periods were chosen and the relevant tests were done through multi-variable and simple regression models by a cross-sectional method for an average period of 10 years. Obtained results showed that ERC portfolio of companies with high reliability was more than that of those with low reliability qualitative characteristic and also were not between relationship cost of capital and the reliability qualitative characteristic.