This study was conducted to find the optimum transformation condition using Agrobacterium harboring promoterless GUS gene. The optimal medium for shoot induction from leaves of Populus nigra×P. maximowiczii was MS medium supplemented with 0.1㎎/ℓ NAA, 0.5㎎/ℓ BAP(94% regeneration frequency and 11.5 average number of shoot) According to the test using pBI121, the concentration of antibiotics for selection marker gene was 100㎎/ℓ kanamycin or 60㎎/ℓ geneticin in the SIM(shoot inducing medium) 3. Two weeks later, callus was induced in the SIM 3 and this callus grew up to 0.5-1cm shoots after 6 weeks in the new SIM 3. And the treatment with methylation inhibitor(5-azacytidine) led to a dramatic increase in foreign gene expression rate from 5.7% to 26.7% . The vector systems showed. different transformation efficiencies based on the fluorometric and histochemical GUS assay. In this study the vector systems used for transformation seemed to affect transformation frequency, in which pEHA101 yielded more transformants(35.9%) than LBA4404/pBI121 did(5.7%). This result indicated that pEHA101 was effective to insert the promoterless foreign gene into a poplar genome.