Nanocomposite bone materials of polymers and hydroxyapatite are widely investigated for bone replacement. The mechanical properties of the composites determine the use of these as implant materials. The molecular phenomenon at the interface between mineral and polymer is known to have significant contribution on overall mechanical response of composites. Understanding behavior of interfaces under applied load, and the load transfer mechanisms will lead to development of superior biomaterial composites with desired properties. We have performed Steered Molecular Dynamics (SMD) simulations on the composite system consisting of hydroxyapatite and polyacrylic acid. Our simulations describe the detailed molecular mechanisms responsible at the interface with applied load. Our SMD simulations also indicate that the polymer shows significant changes when it interacts with the mineral. The load-deformation behavior of polymer has shown that the polymer is stiffer when it is interacting with mineral. The binding and unbinding events are also calculated during load transfer in polymer. This work describes specific molecular mechanism responsible for mechanical behavior in composites used as bone biomaterials.