Purpose: The aim of this retrospective study was to evaluate the relationship between appearance and size of bone bruise and severity of anterior cruciate ligament (ACL) injury with magnetic resonance imaging (MRI), furthermore, to identify the mechanism of ACL injury by analyzing the footprint of bone bruise. Materials and Methods: Sixty-five subjects who was taken an MRI within 6 weeks after acute ACL injury were studied. All MRI showed ACL injury and associated bone bruises. Bone bruises were analyzed using Costa-Paz classification based on appearance and location and also scored using Kornaat bone bruise grading system based on size and location. The severity of the ACL injury was graded using a 4-point system (ie, grade 0-3) with oblique coronal MRI. Results: There was statistically significant correlation between Costa-Paz classification of bone bruises in lateral femoral condyle and grade of ACL injury. There were more type 2 lesions in partial tear group (grade 1, 2), however, type 3 lesions of Costa-Paz classification in complete tear group (P=0.037). Significant positive correlations were observed between Kornaat total bone bruise score and grade of ACL injury (P=0.014). Most common ACL injury mechanism was pivot shift injury. Other mechanisms were clip valgus injury, phantom foot injury, and hyperextension injury. Conclusion: It is highly suggested that appearance and size of bone bruise is related to severity of ACL injury after acute ACL injury. The foot print of bone bruise provides valuable clues to identify ACL injury mechanism.