Ab initio density functional theory (DFT) calculations were performed to examine various factors which may influence the ideal strength, namely multiaxial loading condition and structure with low symmetry. First, the effect of normal stress on the ideal shear strength (ISS) in covalent crystals, Si, C, Ge and SiC, was evaluated. It was found that the response of ISS to normal stress differs depending on the material, while in metals the trend is unchanged. Obtained ISS as a function of normal stress is useful to understand criteria of dislocation nucleation in a pristine crystal because local lattices at the nucleation site undergo superimposed stress components in experiment. Secondly the ideal tensile strength of silicon surface was evaluated to examine how atomistic-level structure affects the mechanical property. The theoretical tensile strength of Si nanofilms with (100) surface, which is flat with dimer-row structures, shows only 20-30% reduction even though the thickness is down to 1 nm, meaning that the flat surface possesses high strength.