  We used merger trees realizations, predicted by the extended Press-Schechter
theory, in order to study the growth of angular momentum of dark matter haloes.
Our results showed that: 1) The spin parameter $\lambda'$ resulting from the
above method, is an increasing function of the present day mass of the halo.
The mean value of $\lambda'$ varies from 0.0343 to 0.0484 for haloes with
present day masses in the range of $ 10^9\mathrm{h}^{-1}M_{\odot}$ to
$10^{14}\mathrm{h}^{-1}M_{\odot}$. 2)The distribution of $\lambda'$ is close to
a log-normal, but, as it is already found in the results of N-body simulations,
the match is not satisfactory at the tails of the distribution. A new
analytical formula that approximates the results much more satisfactorily is
presented. 3) The distribution of the values of $\lambda'$ depends only weakly
on the redshift. 4) The spin parameter of an halo depends on the number of
recent major mergers. Specifically the spin parameter is an increasing function
of this number.
