The aim of this study is to evaluate the Effect of Adding Magnesium sulfate to Lidocaine/epinephrine for Inferior Alveolar Nerve Block(IANB) on Post-operative Pain after Endodontic Treatment in Patient with Irreversible pulpitis. Among the patient who refer to Endodontic Department of Islamic Azad University of Dentistry in Tehran from January to March 2017, 54 patient who have Irreversible pulpitis in lower molars are randomly selected. All patients who will agree to participate in the study and voluntarily sign consent form are randomly divide into 2 groups of 27 patients each. To randomize the patients, each patient is assigned a number. The number in each group are write on paper, and each one will be kept in a separate sealed opaque envelope. Each patient will be asked to choose one of the envelopes and assign to one of the groups. To be Double-blind in this study, operators don’t know the type of anesthetic solution. Patient receive either 0.6ml magnesium sulfate 50% with 1.2ml Lidocaine2%/epinephrine1:100,000 or 0.6ml Normal saline with 1.2ml lidocaine 2%/ epinephrine1:100,000 in one cartridge as the anesthetic solutions for IANB. Each patient will be asked whether they have any signs of soft-tissue anesthesia, if the patients report anesthesia, endodontic treatment commence. Two-visit Root Canal Treatment(RCT) is done in all cases. After first visit, each patient records his pain score on a visual Analogue Scale(VAS) to rate their pain at 6, 12, 24, 48 hours after RCT. The patients are instruct that they can use analgesics (Ibuprofen 400mg) if they feel pain; however, they are also require to record the number of analgesic tablets on their VAS form. Patient with Systemic diseases and addict wiil be Excluded.