The pair-correlation function g(r, t) and its Fourier transform, the structure factor S(q, t), are computed during the gelation process of identical spherical particles using the diffusion-limited cluster-cluster aggregation model in abox. This numerical analysis shows that the time evolution of the characteristic cluster size ξ exhibits a crossover close to the gel time t g which depends on the volumic fraction c. In this model t g tends to infinity when the box size L tends to infinity. For systems of finite size, it is shown numerically that, when t < t g , the wave vector q m , at which S(q, t) has a maximum, decreases as S(q m , t) −1/D , where D is an apparent fractal dimension of clusters, as measured from the slope of S(q, t) . The time evolution of the mean number of particles per clustern is also investigated. Our numerical results are in qualitative agreement with small angle scattering experiments in several systems.