This study includes two main components: the first screening phase and the second clinical intervention phase. During the screening phase, subjects with poor response to clozapine are carefully evaluated by chart-review and clinical assessment. Via chart-review, clinical data of diagnosis, disease course and previous treatment outcome, and concomitant psychotropic agents are obtained. Clinical phenomenology, including psychopathology, psychotic and mood symptoms severity and side effect of psychotropic medication are assessed by clinical interview and observation, which are conducted by experienced clinicians. Blood sample will be also obtained from the subjects for measuring the baseline clozapine drug level and extracting DNA for further analysis. Subjects fulfilling the criteria of treatment-resistant schizophrenia with poor response to adequate dose and duration of clozapine treatment are eligible for the clinical trial phase of 14-week randomized, placebo-controlled amisulpride add-on study. In this phase, subjects are randomly allocated to amisulpride augmentation treatment group and placebo treatment group. Subjects in the former group will receive clozapine and amisulpride combination treatment, while in the latter group will receive clozapine and placebo. Outcomes of clinical efficacy and safety are carefully evaluated by experienced and well-trained research stuffs in the 14 weeks of clinical study period.