A method is introduced in which tensile tests can be performed in situ on micromachined structures. The testing equipment consists of a testing unit mounted on a micromanipulator in a scanning electron microscope. The fracture loads of micromachined beam structures made from thick and thin film polysilicon were measured, and the fracture strengths were then calculated via measurements of the fracture surface areas. Characterization of the film materials was also performed with transmission electron microscopy, atomic force microscopy and scanning electron microscopy to locate the critical defects in the materials. The statistical scatter of the fracture strength values was evaluated using Weibull statistics, which yielded the mean fracture strength and the Weibull modulus-a measure of the amount of scattering In this study, for the first time, Weibull theory was also applied to a real micromechanical structure, i.e. standard test results were transformed into expected strength limits of a more complex structure.