Abstract.[Purpose] The purpose of this study was to determine the physical fitness performance and health-related quality of life of patients with nonspecific low back pain (LBP) after a 4-week supervised fitness exercise intervention in addition to routine physical therapy.[Subjects] Twenty-four patients with nonspecific LBP participated in this study.[Methods] All participants completed either an additional supervised fitness exercise along with conventional physiotherapy twice a week for 4 weeks, or conventional physiotherapy only. Physical fitness, visual analogue scale of pain, the modified Roland-Morris Disability scale, and SF-36 assessments were made before and after the intervention.[Results] Significant improvements were found in physical fitness, including trunk flexors/extensors endurance, lower extensor strength, reaction time of the upper extremity, and the body pain domain of SF-36 after fitness exercise compared to the conventional physiotherapy alone. Decreases in pain intensity were found after treatment in both groups.[Conclusions] A 4-week supervised fitness exercise program was effective at reducing pain intensity and alleviating disability. It also improved trunk muscle endurance, lower extensor strength, and the body pain domain in health related quality of life of patients with nonspecific LBP.