An improved theoretical multiplicity function by means of the excursion set approach is found. The barrier shape used implicitly takes into account the total angular momentum acquired by the proto-structure during evolution and the non-zero cosmological constant. The theoretical multiplicity function obtained in the present paper is compared with analytical and numerical multiplicity functions of other authors. It is shown that when one takes into account the total angular momentum., acquired by the proto-structure during evolution, the multiplicity function obtained is in a better agreement with Yahagi, Nagashima & Yoshii (2004) simulations than with the results of other previous models. Differently from some previous multiplicity function models, the theoretical mass function of the present paper is obtained from a sound theoretical background and not by a simple numerical fit. However. the current level of disagreement between different simulations is not sufficient,to favor one mass function over another when the differences are at the 10 percent level. Hence, at the present time all of these mass functions are essentially equivalent.