The synthesis of β-(1→4)-linked hexa- to octamannoses and their partially acetylated derivatives was efficiently carried out by assembly of appropriate oligomeric fragments using β-selective glucosylation followed by gluco to manno epimerization at a late stage of the synthetic pathway. In the course of this study, we also observed that 2-O-acetylated oligomannoses coexisted in equilibrium with the 3-O-acetylated isomers due to intramolecular migration of the acetyl group. Bioactivity of the synthetic oligomannoses and partially acetylated derivatives was investigated in order to identify the possible smallest oligomer for induction of cytokines as that shown in the polysaccharides extracted from Dendrobium huoshanense.