Objective: This study sought to assess the success rate, effect on blood pressure, and pain of intraosseous injection (IO) and inferior alveolar nerve block (IANB) for pulpal anaesthesia of mandibular posterior teeth with symptomatic irreversible pulpitis as the primary anaesthetic technique. Materials and methods: This randomized clinical trial (IRCT2013022712634N1) was conducted on 60 patients between 18 and 65 years suffering from symptomatic irreversible pulpitis of a mandibular posterior tooth. Patients were randomly divided into two groups. Group one received IO while group two received IANB with 3% mepivacaine. After anaesthetic injection, success rate of pulpal anaesthesia was assessed by pulp testing in the two groups. Systolic and diastolic blood pressures of patients were compared before and after the anaesthetic injections. Level of pain during injection was scored using a visual analogue scale. The data were analyzed using SPSS version 20, t-test and chi square test at p ¼ .05 level of significance. Results: Success rate of IO (56.7%) was significantly higher than that of IANB (23.3%) (p ¼ .008). There was no significant difference in pain during anaesthetic injection (p ¼ .304) or change in systolic (p ¼ .80) and diastolic (p ¼ .28) blood pressures following injection between the two techniques. Conclusions: IO had a higher success rate than IANB for pulpal anaesthesia of mandibular posterior teeth with symptomatic irreversible pulpitis. Neither technique provided profound pulpal anaesthesia.