Hypotactic structure translation, essentially a discourse-level question, is a big challenge in Chinese-to-English (C-E) machine translation (MT). This paper explores the assessment of translation quality of hypotactic structure for C-E MT. We argue that translating a hypotactic structure in C-E translation is essentially transforming a Chinese discoursal semantic hypotactic structure (DS-hypotactic structure) to an English syntactic hypotactic structure (S-hypotactic structure). We propose a scheme to identify the Chinese DS-hypotactic structure and the corresponding English S-hypotactic structure, and formulate the assessment criteria. We then assess the translation quality of current MT systems. The results show that MT technology is still far from satisfactory in translating hypotactic structures. Finally, we discuss the criterion for identifying the Chinese DS-hypotactic structure and reasons and solutions of this MT problem.