Owing to importance of document copyright the need to detect document duplication and plagiarism is increasing. Many studies have sought to meet such need, but there are difficulties in document duplication detection due to technological limitations with the processing of natural language. This thesis designs and implements a discriminator of similar documents with natural language processing technique. This system discriminates similar documents using morphological analysis, syntactic analysis, and weight on low frequency and idiom. To evaluate the system, we analyze the correlation between human discrimination and term-based discrimination, and between human discrimination and proposed discrimination. This analysis shows that the proposed discrimination needs improving. Future research should work to define the document type and improve the processing technique appropriate for each type.