In this study, we developed a one-step, washing-free and amplification-free assay for protein analysis using gold nanoparticle probes (GNPs) and dynamic light scattering (DLS) technique. The target protein concentration was determined by analyzing the level of GNP aggregation caused by antibody-antigen interactions using DLS. Two formats of assays were designed for mouse IgG detection. In the first format of assay, mouse IgG was directly mixed with GNPs conjugated to goat anti-mouse IgG. Due to the multiple binding sites of primary mouse IgG by the secondary antibody, mouse IgG caused nanoparticle aggregation. Mouse IgG can be detected at a concentration as low as 0.5 ng/mL and the dynamic range of this assay is between 0.5 and 50 ng/mL. A second format of assay developed in this study is a competitive assay conducted by using both mouse IgG and goat anti-mouse IgG conjugated GNPs. In this assay format, mouse IgG was detected within a dynamic range of 100 ng/mL to 10 microg/mL. The CV% of these assays is generally well within 10%. In conclusion, we demonstrated here that by using GNPs as a light scattering enhancer and selecting the proper assay formats, low cost, easy-to-conduct, and highly sensitive bioassays can be developed for protein detection and analysis.