Viral nervous necrosis (VNN) has been reported to infect larvae and juvenile of humpback grouper, and until now, this virus is one of main problem for humpback grouper. Co-agglutination test proved to be a simple, rapid and reliable diagnostic test suitable for use in the field or laboratory without any special apparatus. The study aimed to study application of a co-agglutination test using Staphylococci sensitized with specific antibody for the diagnostic of VNN in grouper. First, brain and eye organ samples from diseased fish are homogenized with phosphate buffer saline (PBS) of pH 7.2. Then, the supernatant is collected after centrifugation at 8,000 rpm for 20 min, and finally one drop of the supernatant and one drop of anti VNN antibody sensitized Staphylococci suspension are mixed on a glass slid and observation of the agglutination is performed after 5-10 min. The result shows that co-agglutination technique detects positive VNN in the brain and eye samples. The co-agglutination technique may provide valid result in a very short time as compared with complex method, such as enzyme-linked immunosorbant essay (ELISA) and fluorescient antibody technique (FAT) that require a high cost. Thus, this test can detect VNN faster, simple and more economic.