I13 is a 250 m long hard X-ray beamline for imaging and coherence experiments at the Diamond Light Source. The beamline comprises two independent experimental branches: one for imaging in direct space using X-ray microscopy and one for imaging in reciprocal space using coherent imaging techniques. The mechanical stability is very important for implementation of increased capabilities at latest generation of long beamlines. Therefore, the beam stability monitoring is essential part of the day-to-day operation of the beamlines as well as for analysis of mechanical instability sources for the Diamond II upgrade. In this paper we present the setup developed to measure mechanical stability of beamline based on optical autocollimator.