OsGW2 (RING type E3 ubiquitin ligase) plays important roles in regulating rice yielding traits. According to the OsGW2 cDNA sequences, the homolog of OsGW2 was isolated from Arabidopsis thaliana by RT-PCR and designated as AtGW2. The predicted AtGW2 protein has 401 amino acids, and has a RING-type domain. The grain size and kilo kernels weight of wild-type Arabidopsis plants were lower than those of AtGW2 RNA interference transgenic plants. These results suggested that AtGW2 negatively regulates the grain size and weight in Arabidopsis.