Studying the ideal strength of nanostructured materials is important for understanding of their mechanical properties. We have performed ab initio modeling of nanoscale Si films with (100) surfaces and computed the surface energy and surface stress as well as the ideal tensile strength of the structure. The strength of the thin film is not significantly decreased down to a thickness of less than 1nm. Surprisingly, there is also no considerable effect of the surface reconstruction. This suggests that the lower stresses found for surface crack nucleation in an experiment are due to substantial flaws and not an intrinsic effect of the ideal surface. The band gap energy of the thin film is lower than that of the bulk, but a band gap remains up to high strain in the thin film. © 2005 The American Physical Society.