Cationic peptides were used as coagulant of gold nano-particle (GNP, 20 nm) for the monitoring cellular protein kinase activities. If cationic peptides, which are designed as specific substrates to target protein kinases, were added into the GNP dispersion, the color of the dispersion changed from red to blue due to the aggregation. On the other hand, if the peptide was phosphorylated with target protein kinase, addition of the phosphorylated peptide didn't change the color of the GNP dispersion. Thus, this system can be a rapid assay of protein kinase activity. The method was applicable to the assay of intracellular protein kinase by using cell lysate. In addition, we applied the system for high-throughput screening of protein kinase inhibitor using 1300 kinds of chemical library. The result indicated that the method was also useful for the drug discovery.