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This paper seeks to study, by industry type, the effect of capital structure on the profitability of the firms accepted in Tehran security exchange. To do this, we studied the firms whose financial statements (balances and inane statement) and required data were available for the period (1375-1379). A total number of 108 fi9rms from different industries were selected. Then we surveyed the data …….. the average of liability asset ratio and the return on equity ratio and the average of liability – asset ratio and the return on assets ratio for the five year period on a centralized and annual basis. Simple regression and correlation coefficient were used to test the hypotheses, and their significance was determined by using T and Z statistics. The results suggest that:
a) In general, there exists a positive relationship between capital structure and profitability, but this relationship is statistically poor.
b) The relationship between capital structure and profitability is also depended on the type of industry.
c) The relationship between capital structure and profitability in different industries depends on the definition of profitability.