We have investigated deuteron‐deuteron fusion induced by impact of (D2O)n+and (H2O)n+clusters with n=1–150 on deuterated polyethylene targets at energies of 135–225 keV. Both the energy dependence and magnitude of the fusion yield measured with (D2O)115+clusters confirm the results of Beuhler et al. The size dependence of the (D2O)n+fusion yields measured at the impact energy of 225 keV for small (n<10) clusters follow the ‘‘thermonuclear’’ model proposed by Carraro et al. However, for n≳10 the size dependence rapidly deviates to higher yields from the model as n increases. For H2O and (H2O)2+at 225 keV, the yields roughly agree with the ‘‘knock‐on’’ model by Carraro et al. No fusion event was observed for (H2O)n+clusters with n=4–50, however, n=115 clusters produced an observable fusion rate. The ratio between fusion rates of (H2O)115+and (D2O)115+is 5±2%, in close agreement with the result, ∼5%, measured by Beuhler et al. at 300 keV.