Wood formation in trees requires carbon import from the photosynthetic tissues. In several tree species, including Populus sp. the majority of this carbon is derived from sucrose transported in the phloem. The mechanism of radial sucrose transport from phloem to developing wood is not well understood. We investigated the role of active sucrose transport during secondary cell wall formation in hybrid aspen (Populus tremula x tremuloides). We show that RNAi mediated reduction of PttSUT3 during secondary cell wall formation in developing wood caused thinner wood fibre walls accompanied by a reduction in cellulose and an increase in lignin. Sucrose content in the phloem and developing wood was not significantly changed. However, after 13CO2 assimilation the SUT3RNAi lines contained more 13C than wild type in the sucrose containing extract of developing wood. Hence sucrose was transported into developing wood, but the sucrose derived carbon was not efficiently incorporated to wood fibre walls. A YFP:PttSUT3 fusion localized to plasma membrane suggesting that reduced sucrose import into developing wood fibres was the cause of the observed cell wall phenotype. The results show the importance of active sucrose transport for wood formation in a symplasmically phloem loading tree species, and identify PttSUT3 as a principal transporter for carbon delivery into secondary cell wall forming wood fibres.