Using low temperature X-ray powder diffraction the thermal expansion of YMn2H1 has been determined in the temperature region down to 4.2 K. A sharp increase in a(T) appears when cooling through the ordering temperature (TC = 200 K). Within the accuracy of our experiment no change of the cubic symmetry below TC is observable. A comparison of our results with what has been found for pure YMn2 reveal that above TC a(T) decreases not as fast as in YMn2 with temperature. Furthermore the step -like increase of the cubic lattice parameter at the magnetic ordering temperature is about twenty times smaller in YMn2H1. In our discussion we refer the deviating behaviour of YMn2H1 compared to YMn2 to the suppression of the spin -fluctuation in YMn2H1. The hydrogen atoms cause an increase of the Mn - Mn distances and thus the formation of local Mn -moments is favoured.