The paper is devoted to vibration of the foundations for rotary screw compressors used for gas compression for thermo power plant installed on skid mounting. To evaluate the compressor vibration according to the industry standard VDI 3836, the user should decide whether the foundation is rigid or resilient. The foundation is rigid, if the vertical natural frequency of the foundation is at least 25 % higher than the excitation frequency. The excitation frequency, compressor running speed in Hz is normally known, while the natural frequency is usually not known. Therefore the goal of the study was to find natural frequencies of the skid using analytical calculations, SolidWorks simulation and “bump testing” on site. In the results section it is shown that vibration in axial direction is usually higher for screw compressors than in radial direction, therefore the mathematical model with compressor shaking horizontally was adopted for this study. Limitations of the bump test were discovered. The examples of distinctive and indistinctive spectrum were presented. It was concluded that for the structure of screw compressor foundation the bump testing on site is rather an ineffective tool to detect natural frequencies, because there are too many frequencies appearing in the spectrum and no natural frequencies could be distinguished from the time waveform. Therefore, testing of the equipment foundation natural frequencies has to be performed in the manufacturing facility before installing the equipment and filling the frame with concrete.