The full length ORF of AtPUB18 was cloned from Arabidopsis thaliana by RT-PCR.Informatic analysis manifested that AtPUB18 shared similarity of 74.9% and identity of 63.5% with AtPUB19 using VectorNTI and MEGA5.0;A 1 974 bp promoter fragment of AtPUB18 was fused with GUS gene to generate transgenic A.thaliana.The expression level of AtPUB18 was elevated after drought and cold treatment by histochemical GUS assay.AtPUB18 was fused with green fluorescent protein(GFP) gene to generate a transient expression vector.AtPUB18-GFP fusion protein was expressed in A.thaliana protoplasts and observed using confocal microscope.The result showed that AtPUB18-GFP fusion protein distributed in nucleus and cytosol;AtPUB18 was fused with maltose binding protein(MBP) gene to generated AtPUB18-MBP fusion protein in E.coli for ubiquitin ligase activity assay.The result indicated that AtPUB18 had ubiquitin ligase activity with the presence of wheat E1 and human E2.Our studies implied that AtPUB18 was a functional E3 ligase and localized in nucleus and cytosol,and might be involved in the response to abiotic stress in A.thaliana.