BACKGROUND Femoral nerve block (FNB) with varying concentrations of bupivacaine is often used for postoperative analgesia after anterior cruciate ligament (ACL) reconstruction. OBJECTIVE To determine whether FNB using 0.25% or 0.5% bupivacaine provided better analgesia with less effect on quadriceps strengths after ACL reconstruction. MATERIAL AND METHOD One hundred patients were randomized to receive FNB with 20 mL of 0.25% or 0.5% bupivacaine. Data regarding demographic, effectiveness of FNB, time to first pain, time to first analgesic, pain scores, morphine use, and recovery of sensory and motor function were recorded. RESULTS Median time to first morphine requirement was 12 hours in 0.5% bupivacaine group and 10 hours in 0.25% bupivacaine group (p = 0.048). Pain score at 18 hours was lower in 0.5% bupivacaine group compared with 0.25% bupivacaine group (p = 0.001). When specify to the patellar tendon graft subgroup, the patients requiring morphine were 70% in 0.5% bupivacaine group and 90% in 0.25% bupivacaine group (p = 0.03). No differences were found in demographic data, effectiveness of FNB, time to first pain, morphine consumption, and recovery of sensorimotor function. CONCLUSION FNB with 0.5% bupivacaine provided longer time to first analgesic and lower narcotic requirements after patellar tendon graft ACL reconstruction when compared to 0.25% bupivacaine. Both concentrations showed similar effect on quadriceps strengths.