The stress is an important parameter of nano-thin film of the micro-structure. It is essential for the successful design and operation of many micro-machined devices. In this paper, the experiment idea that using Raman spectrometer to quantitate the stress in the nano-thin film of MEMS was put forward and the formula of the stress of Si and GaAs crystal was derived. In the experiment, the Si nano-thin film and AlAs/GaAs nano-thin film on GaAs substrate were grown by MBE (molecular beam epitaxy). The uniaxial pressure is exerted to nano-thin film by using the pressurization instrument designed by ourselves. Raman spectrums of the nano-thin film of MEMS are measured with the pressure exerted varying. Through the processing of the experiment data, the error of measurement to stress in nano-thin film is maximally 0.9% to Si and 7.5% to GaAs. So, Raman spectrum can be used to accurately quantitate stress in nano-thin film of MEMS in order to assessment of the reliability of micromachined structure. And the method can be applied to quantitate the stress in the experiment of testing the piezoresistor effect of double-quantum-well nano-thin film.