Papillary thyroid carcinoma (PTC) is the most common malignancy of thyroid gland originating from thyroid follicular cells. PTC has generally a good survival rate; however the cases having certain clinico-pathological parameters have poorer prognosis. A population-based cohort study showed that incidence of total thyroid cancers increased from 3.6 per 100000 in 1973 to 8.7 per 100000 in 2002. No significant change was reported for incidence of the less common types including follicular, medullary, and anaplastic carcinomas. In other words the entire increase is attributable to an increase in incidence of PTC, which increased from 2.7 to 7.7 per 100000. Crude death rate of PTC was approximately 0.5 deaths per 100000 individuals from 1973 to 2002. Point mutations in proto-oncogenes including rearranged during transformation (RET), BRAF, V-Raf and rat sarcoma viral oncogene homolog (RAS) were frequently observed in PTC. microRNAs (miRs) are endogenous single stranded non coding RNAs that bind to the 3' non coding region of the target mRNAs, resulting in their selective degradation or inhibition of translation. Therefore miRs are involved in regulation of biological functions. Let-7 family is known as the first group of discovered miRs in human. It has been observed that down regulation of these miRs is associated with malignancies. Let-7 miRs bind to RAS oncogenes and result in their down regulation. Therefore they have been investigated in PTC. In addition, this family has been observed in normal thyroid gland suggesting that it may play a role in function of thyroid gland.Other than let-7 family, other miRs were notable. MiR-146b regulates signal transduction of transforming growth factor-beta (TGF-β) and therefore may play a role in PTC. In addition, oncogene activation has resulted in increase in expression of miR-146b. It has been observed that patients with BRAF mutation have a higher expression level of miR-146b in comparison to patients with wild type of BRAF. MiR-34b is involved in oncogenesis. In has been observed that it was down regulated in cancer cells. A study on different cancer cell lines showed that many genes such as RAS family are targeted by miR-16. MiR-877 is another cancer related miR which is down regulated in hepatocellular carcinoma via targeting cyclin dependent kinase (CDK) 14. Previously diagnostic role of miRs in PTC have been investigated. However, other than diagnosis, it is important to have ability to differentiate aggressive cases from the non-aggressive ones. Therefore, the present study was designed to investigate the association of let-7f, miR-146b-5p, miR-34b, miR-16 and miR-877-5p expression in blood serum and tumor tissue with aggressiveness of PTC in an Iranian population.