In this communication, we find that the single-walled carbon nanotubes (SWNTs) can demonstrate an excellent ssDNA concentration-dependent surface enhanced Raman scattering (SERS) effect after the decoration of DNA-templated in situ grown AgNPs on the surface. Inspired by this, the SWNT@AgNPs hybrid nanocomposite was employed to achieve microRNA detection which is also sensitive to crude extraction from human breast cancer cells and even patient tissues before and after chemotherapy.