To increase our understanding of the tension wood formation and relative gene expression in Populus simonii ×P. nigra, we isolated the immature xylem from both tension wood (TW) and opposite wood (OW) of bent poplars after the species growth were affected by simulated gravity. Two cDNA libraries were then constructed and used to generate 6 048 expressed sequence tags (ESTs), which represented 5 007 unique transcripts and involved 437 ESTs in tension wood formation. The EST distributions were compared between two libraries. The results showed that genes involved in cellulose biosynthesis, cell wall proteins and transcriptional factors were found and expressed at high levels in TW, while genes involved lignin biosynthesis were expressed at high levels in OW. Moreover, genes involved in signal transduction and carbohydrate metabolism were also identified and had different expression models in TW and OW.