In this paper, I describe a study conducted to explore the effects of differences and similarities between L1 and L2 on acquiring English tense and aspect by Chinese learners through the investigation of their interlanguage phenomena. The subjects, twelve Mandarin-Chinese-speaking learners of English, who were from Beijing, Shanghai, Hangzhou, Shengzhen, Hong Kong, and Taiwan, were in graduate schools or postgraduate programs, collages, and a language institute in the United States. The results of the study, with its data base of 2331 subject verbs, indicate that the interlanguage of tense and aspect produced by Chinese learners can be "invaded" by the first language when L1 (Chinese) and L2 (English) are different. However, the similarities between the two languages tend to cause learning difficulties in acquiring English tense and aspect, a finding which, in fact, may challenge the commonsensical belief that where two languages are similar, positive transfer occurs.