Primary Ewing's sarcoma of the mobile spine is rare, but nevertheless, is the second most common primary malignant bone neoplasm in children. Furthermore, in contrast to long bone involvement, delays may occur because symptoms may not be present until neurological deficits occur. Here, we report a case of an 8-year-old boy with lower back pain and radicular pain on the posterior lower extremities with tingling and progressively reduced sensation in both feet. The patient initially seemed to have muscle sprain but was eventually diagnosed with Ewing's sarcoma of the L5 vertebra with intraspinal extension.