To better understand the mechanism of structural instability at the atomistic level, we have proposed a rigorous method of instability mode analysis by solving the eivgenvalue problem of the Hessian dynamical matrix and applied it to dislocation nucleation from surface steps and interfaces. In dislocation nucleation from an interface edge, relation between instability at a low but nonzero temperature and instability modes for vanishing temperature is investigated. An instability mode corresponding to an eigenvector that has a small eigenvalue is activated by thermal fluctuation, resulting in deformation indicated by the eigenvector. This is in contrast to the case of dislocation nucleation from a surface notch, where instability occurrs along a mixed mode. In the case of dislocation nucleation from surface steps, we discuss the area of influence from a step by examining the distribution of eigenvectors utilizing the quasicontinuum (QC) method.