One of the differences between Chinese and English is that the former allows both nullsubjects in finite sentences and null objects, but the latter allows neither. This cross-linguisticvariation is believed to be related to the underspecification of I and topic drop in Chinese but notin English. This paper reports on an empirical study investigating the unlearning of null subjectsand null objects by 159 Chinese learners in their L2 acquisition of English. In L1 acquisition, ithas been found that English-speaking children display an asymmetry by frequently allowing nullsubjects but rarely null objects. The results of this study indicate that there is an asymmetry inChinese learners' L2 English, which, however, is opposite to that found in English L1acquisition: Chinese learners are able to reject the incorrect null subject in English, but unable todetect the ungrammaticality of the null object. It is proposed that the unlearning of null subjectsby Chinese learners of English is triggered by the evidence in their input indicating thespecification of AGR(eement) and T(ense) in English, and that the difficulty in the unlearning ofnull objects is related to the lack of informative evidence to unset the [+ topic-drop]setting in Chinese learners' L2 English.