A quantitative study of the trunk muscle strength was performed in patients with chronic low back pain using isokinetic dynamometer and compared with healthy controls to determine the role of trunk muscles in low back pain. Most patients demonstrated decreased peak torque and total work of trunk muscles and weakness was more outstanding in trunk extensors than flexors and in women than men. The total work % by body weight and trunk flexors/extensors ratio of total peak were increased. In conclusion, the patient with low back pain had a greater weakness in trunk extensors and experienced more pain on trunk extension.