The aggregation of Li, Ti decorated ethylene molecules and its effect on hydrogen storage capacity have been thoroughly studied using a first-principles calculation. The results indicate that the aggregation of C2H4-metal complexes substantially impacts on the hydrogen storage capacity. The aggregation of C2H4-Li reduces the hydrogen storage capacity greatly and makes the hydrogen storage capacity of C2H4-Li complexes hardly meet the demands of the U.S. DOE. The C2H4Ti2 dimers can uptake hydrogen as much as 10 wt%, which suggests that the C2H4Ti2 dimer can be a promising candidate for practical applications.