Clinical assessment of functional disability is an integral part of management in patients with low back pain (LBP). The range of spinal motion is one of LBP disability measure. The aim of this study was to investigate the validity of spinal range of motion as a predictable measure of disability and to analyze the intrarater reliability of back range of motion (BROM) instrument for measurement of active lumber spine range of motion. Forty men patients with chronic low back pain over 6 month’s duration were participated in the study. Their ages ranged from 20 to 40 years. Lumber range of motion was measured with BROM device and disability was evaluated by self reported Roland Morris disability questionnaire (RMDQ). Data were analyzed using Spearman’s correlation, multiple regression analysis models and ICC. Statistical analysis revealed that there was a highly significant moderate to good relation between forward trunk flexion and RMDQ score (rho = −0.59, p < 0.001). While there was a weak correlation between trunk extensions, lateral trunk flexion and trunk rotation with the RMDQ scores (p > 0.05). The main predictors of disability were forward and lateral trunk flexion. Furthermore, intrarater reliability for forward trunk flexion was good (ICC, 0.84), for extension was high (ICC, 0.91), for rotation was good (ICC range, 0.86–0.88), and for lateral flexion was good (ICC range, 0.81–0.82). It was suggested that spinal ROM do not appear to be a valid measure for prediction of the functional disability in patients with chronic low back pain.