Boron nitride nanotubes (BNNTs) have been utilized to strengthen various engineering materials especially metal matrix composites thanks to their extraordinary high tensile strength, elastic modulus, and failure strain. In this paper, single- and multi-walled BNNTs were therefore used to combine with aluminum (Al) metal matrix. Mechanical characteristics and deformation mechanism of nanocomposites reinforced with long (continuous) and short (discontinuous) BNNTs were then investigated for different loadings including uniaxial tension and compression and different boundary conditions based on molecular dynamics simulations. It was found that long BNNTs remarkably improved tensile mechanical properties of the matrix and effectively enhanced elastic modulus and strength of the nanocomposites by ۸۲% and ۷۹.۴%, respectively. They could provide effective barriers to propagation path of dislocations formed inside the matrix. Diameter and wall number of the reinforcement did not leave considerable impacts on the nanocomposite behavior while its atomic fraction remarkably influenced the material response.