Anti N-methyl-D-aspartate antibody encephalitis (Anti-NMDA antibody encephalitis) develops when such antibody binds and damages NMDA receptors in the brain. Psychiatric symptoms and signs,such as hallucination, acute psychosis and delusional state, are often found as early clinical presentations,rather than brain lesion that can be found on neuroimaging. The CSF and EEG studies are often normal, thus such cases are easily misdiagnosed as acute psychosis, neuroleptic malignant syndrome (NMS), viral encephalitis, substance abuse, or malingering. In fact, the symptoms and signs of anti-NMDA antibody encephalitis and NMS are almost identical, making it especially difficult to diagnose. From Since 2007, thanks to the advances of Immunology, such autoimmune encephalitis is now able to be diagnosed accurately. In the past, lots of victims were treated as psychotics when anti-NMDA antibody was not able to be detected. In this case report, we illustrated the clinical presentation, disease progress, responses to treatment, and how the final diagnosis of anti-NMDA encephalitis was made on a 19 years old female who developed acute psychosis.