The purpose of this study is to compare the effect of lumbar stabilization exercises with Mckenzie's exercises on lumbo-pelvic stability and transverse abdominis and multifidus muscles thickness in patients with non-specific chronic low back pain. This clinical trial will be performed at Tehran University of Medical Sciences, in 2012. Thirty patients with non-specific chronic low back pain will be selected through simple non-probability sampling. Inclusion criteria include: non-specific low back pain for a period of at least 3 months and resulted in dysfunction, chronic low back pain with or without referring to lower limb, and age between 18 and 50 years. Exclusion criteria include: recent fracture in lumbar, disc herniation, inflammatory diseases, spondylolisthesis or spondylolysis, malignancy, pregnancy, and exacerbation of symptoms (pain and disability) following exercise performance. Subjects are randomly assigned (random number sequence) to either the stabilization (N=15) or the Mckenzie's exercises group (N=15). Before and after intervention, pain (ordinal) through Visual Analogue Scale (VAS), lumbar lordosis (degree) with flexible ruler, range of lumbar flexion and extension (centimeter) using Modified-Modified Schober test, disability (ordinal) with The Functional Rating Index (FRI), lumbo-pelvic stability using pressure biofeedback, and transverse abdominis and multifidus thickness with ultrasonography will be assessed. Exercise program will be included eighteen sessions of exercise during 6 weeks, three times per week, and one hour per session for both groups.