*Background and objective:Spinal anesthesia is type of regional neuraxial anesthesia that is used for lower abdominal, lower limb, and perianal surgeries. The aim of this study is to evaluate the lower effective isobaric bupivacaine dose in spinal anesthesia for perianal surgery that produces the least level motor blockade. * Patients and methods: Fifty-seven patients were enrolled in this study in Baqubah Teaching Hospital between 10 th of February 2017 to 15 th of September of the same year, all of them underwent perianal surgeries under spinal isobaric bupivacaine anesthesia. They were classified into three groups according to isobaric bupivacaine dose from 10 mg (2 ml) to 5 mg (1 ml) by decreasing 2.5 mg (0.5 ml) in each group. Lumbar puncture was done at the L4-L5/L5-S1 midline intervertebral space with spinal needle G22 Quincke type and isobaric bupivacaine dose was injected in sitting position then immediately turned the patient to supine positioin. A successful block was defined as one that was sufficient to proceed with surgery without any supplementation other than premedication. *Results: Different doses of isobaric bupivacaine spinal anesthesia result in different levels of spinal blockade(sensory and motor) which proportionates directly to the dose, while 10 mg isobaric bupivacaine causes spinal blockade up to T12 dermatome, 5 mg dose causes blockade just to L3 dermatome in most cases. 5 mg isobaric bupivacaine spinal anesthesia provides adequate surgical analgesia , good muscle relaxation, absence of reflexes to surgical stimuli with least level motor spinal block over sacral nerves that innervate perianal area. Conclusion: 5 mg isobaric bupivacaine is the lower effective dose in spinal anesthesia that provides good operating conditions, rapid recovery, early discharge, no postoperative anesthetic complications with most lower level motor blockade.