Different curdlan sulfates were synthesized by modification of curdlan with sulfur trioxide-pyridine complex, and the one with the highest sulfur content (9.23 ± 0.16) %, CS3, was used to study the immunomodulating effect on murine RAW264.7 macrophages and bone marrow derived dendritic cells (BMDCs). The results showed that the treatment of macrophages with CS3 could not only increase the nitric oxide (NO) release and the cytokines TNF-α, IL-6 and IL-1β production significantly, but also enhance the inducible NOS (iNOS) expression, NF-κBp65 nuclear translocation, Erk1/2 and SAPK/JNK phosphorylation. The combination of CS3 with GM-CSF upregulated immature BMDCs to express major histocompatibility complex II (MHCII) and CD11c surface markers, CD40, CD80 and CD86 costimulatory molecules, as well as the cytokines of IL-12p70 and IL-6. The surface plasmon resonance (SPR) analysis found that CS3 exhibited binding affinity against dectin-1. These results suggested that curdlan sulfate can possibly be developed as a new immunotherapy agent and anti-viral vaccine adjuvant.