The deuterium cluster-ion, which consists of three deuterons, was used to study the cluster effect in the D-D fusion reaction in solids. The cluster-ion beam (deuteron-cluster d+3) and isolated deuteron (single deuteron d+), whose energies were adjusted as from 10 to 100keV per deuteron, were alternatively implanted into deuterium-palladium target. The differences of the fusion reaction rates were obtained by comparing the radiation yields of charged particles and neutrons that were induced by two kinds of beams with the same velocity. Experimental results are as follows: Each deuteron of cluster-ion (cluster deuteron) induced higher D-D fusion rate, compared with an isolated deuteron, when the deuteron energy was lower than 40keV per deuteron; while the cluster deuteron induced lower fusion rate when its energy was higher than 50keV per deuteron. The ratio of fusion rate induced by cluster deuteron to that induced by isolated deuteron decreased with the increase of deuteron energy. Some interpretations have been discussed.