The lack of a standardized Thai Language aphasia test raises difficulties not only with the assessment and treatment planning for the clinical but also with the accurate diagnosis and the reliable incidence for research on aphasiology in Thailand. This study aimed to use the Thai version of German Aachen aphasia (THAI-AAT), which is systematically adapted according to well-defined linguistic criteria and psychometric requirement, to assess the language deficit of Thai aphasic patients. The subjects participating in this study were 125 aphasia patients, 60 non-aphasic brain damaged patients and 120 normal subjects. The result revealed that the THAI-AAT is linguistically parallel in test design and fulfills the same psychometric properties as the original. The THAI-AAT obtains the goals: to differential diagnosis of aphasia distinguishing it from non-aphasic disturbance and to identify the type of aphasic syndrome.