On the basis of the some studies recently published on the concentration of potentially dangerous levels of mercury, a global pollutant and highly toxic element in sea fish and specially tuna fish, this work has been carried out to determine the concentration of mercury in canned tuna fish, that compose a large majority of the fish most people eat. For this reason, 30 cans, produced by different factories were randomly collected through the Tehran vicinity and the total mercury contents of all samples was measured with GBC hydride generation atomic absorption spectrometry. To better extraction of total mercury in the biologic samples, the effects of SnCl2 (reducing agant) and sample weight on the rate of mercury absorption was examined either. Statistical analysis was done applying T-test and SPSS (v.10). Based on the results, the tuna fish from different factories contained different concentrations of mercury the average of which was 146.65±63.35 ppb. Also, it was found that sample weight, and not different concentrations of SnCl2 affect absorption. Although the mercury concentrations of the samples tested was below standard or maximum residual level (MRL), due to teratogenic effects of mercury, it is strongly recommended to do screening tests on tuna fish products for domestic consumption or for export purposes.