The width of an unresolved transition array, which was once regarded as insensitive to electron temperature by Audebert et al. [Phys. Rev. A 32 (1985) 409], is found to be a temperature-sensitive function to a certain extent and has the potential of being used in plasma diagnostics. In this letter, on the assumption that the plasma has achieved the local thermodynamical equilibrium and that each of the subarrays has a Gaussian shape, the electron temperature of a gold plasma produced by laser irradiation of a microdot is obtained by comparing the experimental width with the theoretical width. The full widths at half maximum of the unresolved transition arrays are computed in the formalism of the spin-orbit-split arrays.