A low-cost, flexible, and disposable immunosensor is presented in this paper. The single-walled carbon nanotubes (SWNT) and biomolecules are self-assembled on micro patterned electrodes. The immuno-chip acts as the platform of a horseradish peroxidase (HRP) labeled sandwiched Enzyme-Linked ImmunoSorbent Assay (ELISA). The pH change induced by the biochemical reactions influences the electrical conductance of SWNT. The detection resolution of 0.4 ng/ml (2.5 pM) for normal rabbit immunoglobulin G (IgG) is demonstrated. The new fabrication technique and the HRP labeled detection protocol can be extended to the recognition of other antigens for critical applications to clinical diagnosis, food toxin detection, and environment monitoring, etc.