We construct merger trees based on the extended Press-Schechter theory ( EPS ) in order to study the merger rates of dark matter haloes over a range of present day mass ( 10 ^ { 10 } M _ { \sun } \leq M _ { 0 } \leq 10 ^ { 15 } M _ { \sun } ) , progenitor mass ( 5 \times 10 ^ { -3 } \leq \xi \leq 1 ) and redshift ( 0 \leq z \leq 3 ) . We used the first crossing distribution of a moving barrier of the form B ( S,z ) = p ( z ) + q ( z ) S ^ { \gamma } , proposed by Sheth & Tormen , to take into account the ellipsoidal nature of collapse . We find that the mean merger rate per halo B _ { m } / n depends on the halo mass M as M ^ { 0.2 } and on the redshift as ( \mathrm { d } \delta _ { c } ( z ) / \mathrm { d } z ) ^ { 1.1 } . Our results are in agreement with the predictions of N-body simulations and this shows the ability of merger-trees based on EPS theory to follow with a satisfactory agreement the results of N-body simulations and the evolution of structures in a hierarchical Universe .