Plant senescence is a final process of growth and a survival strategy to use limited nutrients efficiently during development and adaptation. Tree, however, runs senescence annually for winter dormancy. Therefore understanding senescence procedure and mechanism is striking issue for tree breeding and wood productivity. In this study, we isolated a SENESCENCE 1 (PagSEN1) gene from Populus alba ⨯ P. glandulosa and determined its expressional characteristics under various conditions. The PagSEN1 encodes a putative 243 amino acid protein containing a rhodanese domain. Southern blot analysis suggested that two copies of the PagSEN1 gene are present in the poplar genome. The PagSEN1 is expressed most strongly in mature leaves but most weakly in roots. The gene is significantly up-regulated by treatment with mannitol, NaCl, ABA and JA, but not by cold, SA and GA3. These results indicate that the PagSEN1 is involved in senescence response induced by natural aging and environmental stresses. This research will provide valuable information for senescence study. To clarify SEN1 gene in poplar, we will make PagSEN1 over-expressed poplar to understand senescence procedure in detail.