The aim of this study was to identify the Potato Spindle Tuber Viroid (PSTVd) by Reverse Transcriptase-PCR. Among all plant diseases which are caused by viroids, PSTVd was the first viroid which is recognized by plant pathologist. Suspended potato tubers from Razavi and Northern Khorasan provinces collected and cultured. Leaves of cultured tubers were used for RNA extraction by PEG6000 Precipitation method. Subsequent RT-PCR reaction has been carried out using specific primers. A 359 bp fragment has been appeared after electrophoresis which has not appeared in healthy tuber samples. Digestion of obtained fragment with BamHI confirmed the sequence by producing two fragments of 119 and 240 bp. Sequencing has been done to certify the PSTVd. The sequencing result showed that infection in tested areas is caused by mild strain of PSTVd. In this project from 250 samples, 14 tubers were infected. This was the first report of occurrence of mild strain of PSTVd in Iran.