MicroRNAs (miRNAs) are a class of non-coding RNA molecules (containing about 22 nucleotides)found in plants, animals and some viruses, that functions in RNA silencing and post-transcriptionalregulation of gene expression. miRNAs represent one of the novel strategies for cancer screening. Atpresent, in studies that focused on the association between miRNAs and tumors, abnormal expression ofmiRNA-155 in patients with breast cancer has have attracted considerable attentions. As an oncogene,high expression of miR-155 was considered as a breast cancer risk factor. Experiments was carried outusing ELISA Plate Reader and nanoparticles size distributions were measured with dynamic lightscattering. Here, we introduce an optical aptasensor based on gold nanoparticles and MnCl2 salt. In thissimple method, detection of miRNA-155 was done with increasing of MnCl2 salt to a solution containinggold nanoparticles that conjugated with thiolated probe. With this method even 6 pM concentration ofmiRNA-155 were detected. For test selectivity of the system, response of biosensor against three-basepairmismatch, also investigated. Results exhibited high selectivity for miRNA-155 over three-base-pairmismatch.