The antisense Caffeoyl CoA O-methyltransferase (CCoAOMT) cDNA was transformed into Chinese white poplar (Populus tomentosa) mediated by Agrobacterium tumefaciens. Many factors affecting the transformation efficiency were studied and a stable transformation system was established. PCR-Southern blot analysis indicated that antisense CCoAOMT cDNA had been integrated into the genome of the transgenic poplars. RT-PCR and Western blot analyses demonstrated that the endogenous CCoAOMT gene was suppressed at both transcriptional and translational levels. Klason lignin content assay exhibited the lignin reduction to different degrees in transgenic poplars. The stems of partial transgenic poplars with the remarkable lignin reduction turned red, and the color distribution was stripped or spotted. Taken together, these results suggested that CCoAOMT gene would be a potential useful gene in altering lignin biosynthesis by biotechnology for improving wood properties.