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Postero-anterior (PA) mobilization is widely used to manage low back pain by physiotherapists. The PA load is applied through the spinous process of a vertebra. Low bone density is a counter-indication of PA mobilization, whether PA mobilization may cause fractures in fragile vertebrae is unclear. Therefore, the aim of this study was to quantify the role of bone density on a fracture risk in the first lumbar vertebra subjected to PA load. A finite element model of the first lumbar (L1) vertebra of an elderly female was created to predict the fracture risk of the PA mobilization. The von Mises stress and minimum principal strain were used as the assessment indicators. Three different bone density cases were evaluated to reflect healthy, osteoporotic, and severe osteoporotic conditions by assuming heterogeneous moduli based on local bone density converted from computed tomographic images. In the severe osteoporotic condition under PA load, the maximum von Mises stress and largest compressive strain occurred in the pedicles and spinous process. These stress and strain exceeded the yield stress and yield strain indicating a high risk for failure. The resulted stress and strain were also excessive in the pedicles for healthy and moderate osteoporotic conditions. PA mobilization can increase the risk of vertebra fracture in elderly with osteoporosis. The pedicles and spinous process of osteoporotic L1 vertebra are the critical regions prone to fracture. We recommend that it is crucial to be reduce force when applying the PA mobilization to elderly with osteoporosis.