The purpose of the present study is to compare the postoperative pain after the use of rotary ProTaper system to OneShape system in maxillary and mandibular anterior and premolar teeth with necrotic pulps treated in one visit. Forty patients who fulfilled specific inclusion criteria were assigned to two groups according to the root canal instrumentation technique used, OneShape or ProTaper Universal. Root canal treatment was carried out in single visit, and the severity of postoperative pain was assessed by numerical rating scale (NRS) score after the session until complete pain relief was achieved. Analgesic consumption was also recorded. Results: mean of NRS scores between the two groups and within the same group at different follow up periods(6, 12, 24 and 48 hours) showed no statistically significant difference. Also, There was no statistical significant difference in analgesic consumption between the two groups.