The purpose of this study was to compare the effectiveness of the High Flux and Low Flux membranes on parathyroid hormone serum level in patients with secondary hyperparathyroidism undergoing chronic dialysis. Total of 40 patients referring to Tohid Hospital dialysis center in 2014 who passed inclusion criteria of the study were selected and using the table of random numbers randomly divided into two groups of 20. The control group with Low Flux membrane and the intervention group with High Flux membrane. In order to double blind the study, a colleague who had not sensitivity to study toke the blood samples, he also sent the samples to lab. Inclusion criteria included high level serum of parathyroid hormone (greater than 300pg / dl), performing dialysis by fixed means, dialysis three times a week for four hours, the patient should be at least 6 months on dialysis program, previous use of the Low Flux membrane, bearing on dialysis, age greater than 18 years, the ability to participate in the study and exclusion criteria included; patients with acute renal failure, patients with poor vascular access that may affect their quality of dialysis , age less than 18 years, the continued lack of patient cooperation, discontinuation of dialysis for any reason, being dialysis by High Flux membrane previously. In the beginning of the study for both groups blood samples were taken before dialysis. Laboratory evaluation included serum levels of phosphorus and parathyroid hormone were conducted. The use of Low Flux and High Flux membranes in the both groups continued for three months. Laboratory evaluation included serum levels of parathyroid hormone and phosphorus was repeated again In the end of the study for both groups.