Measurements of lattice parameters were made for a single crystal of a high-purity synthetic diamond using the x-ray-diffraction Bond method between 4.2 and 300 K. The key features of the present measurement are as follows: (1) The lattice parameter is determined with an accuracy of ${10}^{\ensuremath{-}6}.$ (2) A very small negative thermal expansion is detected between 30 and 90 K. (3) Thermal expansion shows a behavior of the ${T}^{4}$ law represented by the Debye theory within the measuring temperature range. By comparing the present results with the previously reported results for nitrogen-doped diamond and boron-doped diamond, it is found that diamond becomes harder following doping by nitrogen.