One of the differences between Chinese and English is that the former allows both null subjects and null objects in finite sentences, the latter allows neither. Besides, in Chinese, it has been found that Chinese-speakers display an asymmetry of null objects and null subjects. So far, there have been debates about the property of the null arguments in Chinese. Reviewing the previous studies which take null argument as a pro or a variable, the study concentrates on radical pro-drop theory and analyzes null arguments from two derivations, one as silent pronouns,and the other as the results of ellipsis. As an experimental study that investigates the mental representation of Chinese null subjects and null objects in L2 grammar, the study further tests whether the asymmetry of null subjects and null objects found in Chinese L1 also exists in English speakers’ L2 Chinese.