In this work, we have firstly electrodeposited small gold seeds (average diameter of ~40 nm) onto transparent indium tin oxide (ITO) thin film coated glass. Then silver triangular nanoplates with edge lengths of ~200 nm were fabricated using seed-mediated growth method. The localized surface plasmon resonance (LSPR) peak was located at ~700 nm. Finally, a label-free plasmon immunosensor was prepared by directly immobilizing goat anti-mouse IgG onto silver surface. The performance of the LSPR immunosensor was investigated. The red-shift of the biosensor was linearly proportional to mouse IgG concentration ranged from 5 ng/mL to 500 ng/mL, with a detection limit of 2 ng/mL. The label-free immunosensor was simple, sensitive and selective.