pf40 is obtained from millet unmature seeds cDNA library. It has high similarity with mental transporter family by blasting in DDBJ, GENBANK, EMBL. It is predicted that PF40 is included in ZIP family. Software analysis indicates that PF40 is a membrane protein. But which membrane system it localized in is unclear. A full PF40 cDNA fused at the 5′ end of the green fluorescent protein (GFP) coding sequence was introduced and stably expressed in tobacco by agrobacterium-mediated transformation. The subcellular localization of GFP was analyzed by fluorescence microscope and laser confocal microscope. The fluorescence of PF40-GFP fusion protein was detected mainly in the ER, demonstrating that PF40 was mainly localized in the ER. 4 truncated pf40 fragments were fused with gfp. The truncated fusion genes also were introduced and stably expressed in tobacco. N-terminal amino acid region can locate the protein in the ER.