Diffusion-limited cluster aggregation and gelation are studied using lattice and off-lattice Monte Carlo simulations. The pair correlation function g(r) and the structure factor S(q) of the particle gels were investigated as a function of the volume fraction (05-49%) and time. At volume fractions below 5%, the gel structure is fractal on small length scales with d(f) = 1.8. g(r) shows a weak minimum at the correlation length (xi), before reaching the average concentration at large length scales. The cut-off function of g(r) varies during the aggregation process, but at a given t/t(g), where t(g) is the gel time, it is a universal function of r/xi. At high volume fractions, the structure is dominated by excluded-volume interactions, while at low volume fractions, it is determined by the connectivity.