Introduction & Objective: Low Back Pain (LBP) is among the most common causes for pain and disability to do different tasks. Also, it is one of the common reasons for medical appointments. Magnetic Resonance Imaging (MRI) is a suitable method to diagnose and assess muscular atrophy and lack of attention to atrophy may lead to lack of back pain diagnosis. The objective of the present study is to examine the relationship between muscular atrophy and its severity with pain and disability in patients. Materials & Methods: This analytical cross sectional study was conducted on 288 people above 20 years old during 2016-2018 on three groups (n = 96) including healthy people (control group), people with LBP and people with lumbar discopathy. Parameters of interest in patients included atrophy severity, ratio between CSA of psoas muscles and intervertebral disk (PS: Disk), the ratio of erector spinae muscle and intervertebral disk (Er: Disk), the ratio between multifidus muscle and intervertebral disk (MF: Disk), pain, and anxiety. Results: The ratio of PS: Disk, Er: Disk, and MF: Disk was significantly lower in LBP group. The prevalence of moderate fatty atrophy was significantly higher in LBP group (96.23%). This was not significantly different from other groups of the study. Average Oswestry Disability Index (ODI) was significantly higher in lumbar discopahy. Also, ODI in LBP group as significantly larger than healthy people. Pain duration, axial LBP, and anxiety in LBP group were significantly larger than other groups. Conclusions: According to the results, LBP and para-spinal muscle atrophy in patients with LBP were larger in patients with LBP compared with patients with lumbar discopahy. CSA in patients with lumbar discopahy and healthy patients showed no significant difference. ODI and disability in patients with lumbar discopahy (24.5± 67.164) were larger than other groups.