Carbon nanotubes (CNTs) possess exceptional mechanical properties for engineering applications. CNTs have low bending stiffness and high aspect ratio, which make CNTs easy to agglomerate in polymer matrix. The agglomeration of nanotubes is one of the important factors which affect the effective elastic modulus of nanocomposites. In this study, a new micromechanical model consisting of agglomeration CNTs and pure matrix is applied to analyze the effect of CNTs agglomeration on the effective elastic modulus. The Mori-Tanaka method is employed to calculate the elastic modulus of randomly oriented CNTs reinforced composites. It is noted that the stiffness of nanocomposites is very sensitive to the CNTs agglomeration.