  We study the Hubble parameter $H(z)$ in perturbed Friedmann universe and
obtain an expression of the perturbed Hubble parameter $H(z,\textbf{n})$. We
derive the Hubble parameter power spectrum by using the initial spectrum during
inflation and the Bardeen transfer function. We obtain a semi-analytical
expression in the case of cold dark matter (CDM) universe. Similar with
luminosity distance, the Hubble parameter spectrum is suggested to be an useful
observational tool to determine some cosmological parameters. In addition, we
show that the Hubble parameter power spectrum could be used to check whether
the expansion is accelerated by the second order small scale fluctuation.
