BK virus is a common human polyomavirus typically occurs in early childhood and cause subclinical primary infection. Use of immunosuppressive treatments may lead to reactivation of primary latent infection of BK virus. Nephropathy associated with the BK virus has become an important cause of allograft rejection post renal transplantation. In this research for determination of BK virus reactivation the prevalence of BK virus were evaluated pre and post renal transplantation. Seventy eight renal transplant patients including: 51 males (65.4%) and 27 females (34.6%) aged between 16-60 years (mean = 34.78) were studied. Blood (plasma) and urine samples were collected from renal recipients, pre-transplant and 1 month and 4 months post-renal transplant. The molecular prevalence of BK virus was analyzed by an in-house nested PCR protocol in these samples. The collected data were statistically analyzed with SPSS software. BK virus genome was detected in 5 of 78(6.4%) urine samples pre-transplantation. But BK virus DNA was diagnosed in 10 of 78 (12.8%) and 30 of 78 (38.6%) urine samples in first and fourth months post-renal transplantation, respectively. All pre-transplant collected plasma specimens were negative for BK virus-PCR. But in the first and fourth months post-transplantation, positive results of BK virus-PCR was detected in 1 of 78 (1.3) and 16 of 78 (20.5%) of plasma samples, respectively. Positive results of BK virus-PCR differed significantly between first and fourth months post-renal transplantation. The results of this investigation showed that the prevalence of BK virus reactivation was increased, switching pre to post renal engraftment. Therefore monitoring of BK virus infection was recommended for better management of the surveillance of transplant patients.