Cotton is the most important textile crop due to its cellulose-enriched mature fibres, single celled hairs derived from ovule epidermis at anthesis. Despite the great potential for increasing cotton productivity through genetic engineering of fibre development, little progress has so far been made in this area. This is in sharp contrast to the success of pest and herbicide resistant transgenic cotton that have already made a large impact on agriculture in both the U. S. and Australia (1). The major impendence to fibre engineering is due to our poor understanding of the biology of the cotton fibre, particularly, the identities and functions of genes controlling various fibre development processes. Cotton fibres are metabolically active cells in utilising hexose or its derivates from phloem-imported sucrose for its initiation, elongation and cellulose synthesis (2,3,4,5). Sucrose synthase (SuSy)is the key enzyme in cotton fibre to break down incoming sucrose into fructose and DDP-glucose (2,3). The latter is the immediate substrate for cellulose synthesis (6). However, previous evidence on the role of SuSy in cotton fibre development is largely correlative in nature (2,3,4). Here, by using reverse genetic approach, we have now demonstrated that SuSy indeed plays a crucial role in cotton fibre initiation, elongation and cellulose synthesis.