In this study, the aim is to determine whether adding Memantine to the standard therapeutic regimen of schizophrenic patients could improve some aspects of the symptom burden or not. This study is a 8-week randomized, double-blind, placebo-controlled trial. Fifty patients with chronic stable schizophrenia with residual symptoms are randomly divided into the intervention and control groups. Simple randomization based on a random-numbers table is used to assign subjects to either the placebo or the intervention group. Subjects in the intervention group receive memantine with an initial dose of 10 mg/day for first week, which is increased to 20 mg/day on the second week and continue with the same dose until the end of the study along with their antipsychotic regimen .The control group receive their antipsychotic regimen alone for eight weeks. Positive and Negative Syndrome Scale (PANSS), Clinical Global Impression of Severity (CGI-S), Clinical Global Impression of Improvement (CGI-I), Yale–Brown Obsessive Compulsive Scale (Y-BOCS) and Calgary Depression Scale for Schizophrenia are assessed at the baseline and on the third and sixth and eighth weeks. All schizophrenic patients are managed based on the guidelines of the American Psychiatric Association. The diagnosis is made by two psychiatrists independently, according to the DSM-V criteria .During the study period, only co- medication with anticholinergic drugs and benzodiazepines are allowed.